hadoop-yetus commented on a change in pull request #1196: HDDS-1881. Design
doc: decommissioning in Ozone
URL: https://github.com/apache/hadoop/pull/1196#discussion_r309207176
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File path: hadoop-hdds/docs/content/design/decommissioning.md
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+---
+title: Decommissioning in Ozone
+summary: Formal process to shut down machines in a safe way after the required
replications.
+date: 2019-07-31
+jira: HDDS-1881
+status: current
+author: Anu Engineer, Marton Elek, Stephen O'Donnell
+---
+
+
+# Abstract
+
+The goal of decommissioning is to turn off a selected set of machines without
data loss. It may or may not require to move the existing replicas of the
containers to other nodes.
+
+There are two main classes of the decommissioning:
+
+ * __Maintenance mode__: where the node is expected to be back after a while.
It may not require replication of containers if enough replicas are available
from other nodes (as we expect to have the current replicas after the restart.)
+
+ * __Decommissioning__: where the node won't be started again. All the data
should be replicated according to the current replication rules.
+
+Goals:
+
+ * Decommissioning can be canceled any time
+ * The progress of the decommissioning should be trackable
+ * The nodes under decommissioning / maintenance mode should not been used for
new pipelines / containers
+ * The state of the datanodes should be persisted / replicated by the SCM (in
HDFS the decommissioning info exclude/include lists are replicated manually by
the admin). If datanode is marked for decommissioning this state be available
after SCM and/or Datanode restarts.
+ * We need to support validations before decommissioing (but the violations
can be ignored by the admin).
+ * The administrator should be notified when a node can be turned off.
+ * The maintenance mode can be time constrained: if the node marked for
maintenance for 1 week and the node is not up after one week, the containers
should be considered as lost (DEAD node) and should be replicated.
+
+# Introduction
+
+Ozone is a highly available file system that relies on commodity hardware. In
other words, Ozone is designed to handle failures of these nodes all the time.
+
+The Storage Container Manager(SCM) is designed to monitor the node health and
replicate blocks and containers as needed.
+
+At times, Operators of the cluster can help the SCM by giving it hints. When
removing a datanode, the operator can provide a hint. That is, a planned
failure of the node is coming up, and SCM can make sure it reaches a safe state
to handle this planned failure.
+
+Some times, this failure is transient; that is, the operator is taking down
this node temporarily. In that case, we can live with lower replica counts by
being optimistic.
+
+Both of these operations, __Maintenance__, and __Decommissioning__ are similar
from the Replication point of view. In both cases, and the user instructs us on
how to handle an upcoming failure.
+
+Today, SCM (*Replication Manager* component inside SCM) understands only one
form of failure handling. This paper extends Replica Manager failure modes to
allow users to request which failure handling model to be adopted(Optimistic or
Pessimistic).
+
+Based on physical realities, there are two responses to any perceived failure,
to heal the system by taking corrective actions or ignore the failure since the
actions in the future will heal the system automatically.
+
+## User Experiences (Decommissioning vs Maintenance mode)
+
+From the user's point of view, there are two kinds of planned failures that
the user would like to communicate to Ozone.
+
+The first kind is when a 'real' failure is going to happen in the future. This
'real' failure is the act of decommissioning. We denote this as "decommission"
throughout this paper. The response that the user wants is SCM/Ozone to make
replicas to deal with the planned failure.
+
+The second kind is when the failure is 'transient.' The user knows that this
failure is temporary and cluster in most cases can safely ignore this issue.
However, if the transient failures are going to cause a failure of
availability; then the user would like the Ozone to take appropriate actions to
address it. An example of this case, is if the user put 3 data nodes into
maintenance mode and switched them off.
+
+The transient failure can violate the availability guarantees of Ozone; Since
the user is telling us not to take corrective actions. Many times, the user
does not understand the impact on availability while asking Ozone to ignore the
failure.
+
+So this paper proposes the following definitions for Decommission and
Maintenance of data nodes.
+
+__Decommission__ of a data node is deemed to be complete when SCM/Ozone
completes the replica of all containers on decommissioned data node to other
data nodes.That is, the expected count matches the healthy count of containers
in the cluster.
+
+__Maintenance mode__ of a data node is complete if Ozone can guarantee at
least one copy of every container is available in other healthy data nodes.
+
+## Examples
+
+Here are some illustrative examples:
+
+1. Let us say we have a container, which has only one copy and resides on
Machine A. If the user wants to put machine A into maintenance mode; Ozone will
make a replica before entering the maintenance mode.
+
+2. Suppose a container has two copies, and the user wants to put Machine A to
maintenance mode. In this case; the Ozone understands that availability of the
container is not affected and hence can decide to forgo replication.
+
+3. Suppose a container has two copies, and the user wants to put Machine A
into maintenance mode. However, the user wants to put the machine into
maintenance mode for one month. As the period of maintenance mode increases,
the probability of data loss increases; hence, Ozone might choose to make a
replica of the container even if we are entering maintenance mode.
+
+4. The semantics of decommissioning means that as long as we can find copies
of containers in other machines, we can technically get away with calling
decommission complete. Hence this clarification node; in the ordinary course of
action; each decommission will create a replication flow for each container we
have; however, it is possible to complete a decommission of a data node, even
if we get a failure of the data node being decommissioned. As long as we can
find the other datanodes to replicate from and get the number of replicas
needed backup to expected count we are good.
+
+5. Let us say we have a copy of a container replica on Machine A, B, and C. It
is possible to decommission all three machines at the same time, as
decommissioning is just a status indicator of the data node and until we finish
the decommissioning process.
+
+
+The user-visible features for both of these are very similar:
+
+Both Decommission and Maintenance mode can be canceled any time before the
operation is marked as completed by SCM.
+
+Decommissioned nodes, if and when added back, shall be treated as new data
nodes; if they have blocks or containers on them, they can be used to
reconstruct data.
+
+
+## Mainteneance mode in HDFS
+
+HDFS supports decommissioning and maintenance mode similar to Ozone. This is a
quick description of the HDFS approach.
+
+The usage of HDFS maintenance mode:
+
+ * First, you set a minimum replica count on the cluster, which can be zero,
but defaults to 1.
+ * Then you can set a number of nodes into maintenance, with an expiry time
or have them remain in maintenance forever, until they are manually removed.
Nodes are put into maintenance in much the same way as nodes are decommissioned.
+ * When a set of nodes go into maintenance, all blocks hosted on them are
scanned and if the node going into maintenance would cause the number of
replicas to fall below the minimum replica count, the relevant nodes go into a
decommissioning like state while new replicas are made for the blocks.
+ * Once the node goes into maintenance, it can be stopped etc and HDFS will
not be concerned about the under-replicated state of the blocks.
+ * When the expiry time passes, the node is put back to normal state (if it
is online and heartbeating) or marked as dead, at which time new replicas will
start to be made.
+
+This is very similar to decommissioning, and the code to track maintenance
mode and ensure the blocks are replicated etc, is effectively the same code as
with decommissioning. The one area that differs is probably in the replication
monitor as it must understand that the node is expected to be offline.
+
+The ideal way to use maintenance mode, is when you know there are a set of
nodes you can stop without having to do any replications. In HDFS, the rack
awareness states that all blocks should be on two racks, so that means a rack
can be put into maintenance safely.
+
+There is another feature in HDFS called "upgrade Domain" which allows each
datanode to be assigned a group. By default there should be at least 3 groups
(domains) and then each of the 3 replicas will be stored on different group,
allowing one full group to be put into maintenance at once. That is not yet
supported in CDH, but is something we are targeting for CDPD I believe.
+
+One other difference with maintenance mode and decommissioning, is that you
must have some sort of monitor thread checking for when maintenance is
scheduled to end. HDFS solves this by having a class called the
DatanodeAdminManager, and it tracks all nodes transitioning state, the
under-replicated block count on them etc.
+
+
+# Implementation
+
+
+## Datanode state machine
+
+`NodeStateManager` maintains the state of the connected datanodes. The
possible states:
+
+ state | description
+ ------------------|------------
+ HEALTHY | The node is up and running.
+ STALE | Some heartbeats were missing for an already missing
nodes.
+ DEAD | The stale node has not been recovered.
+ ENTER_MAINTENANCE | The in-progress state, scheduling is disabled but the
node can't not been turned off due to in-progress replication.
+ IN_MAINTENANCE | Node can be turned off but we expecteed to get it back
and have all the replicas.
+ DECOMMISSIONING | The in-progress state, scheduling is disabled, all the
containers should be replicated to other nodes.
+ DECOMMISSIONED | The node can be turned off, all the containers are
replicated to other machine
+
+
+
+## High level algorithm
+
+The Algorithm is pretty simple from the Decommission or Maintenance point of
view;
+
+ 1. Mark a data node as DECOMMISSIONING or ENTERING_MAINTENANCE. This implies
that node is NOT healthy anymore; we assume the use of a single flag and law of
excluded middle.
+
+ 2. Pipelines should be shut down and wait for confirmation that all pipelines
are shutdown. So no new I/O or container creation can happen on a Datanode that
is part of decomm/maint.
+
+ 3. Once the Node has been marked as DECOMMISSIONING or ENTERING_MAINTENANCE;
the Node will generate a list of containers that need replication. This list is
generated by the Replica Count decisions for each container; the Replica Count
will be computed by Replica Manager;
+
+ 4. Once the Replica Count for these containers go back to Zero, which means
that we have finished with the pending replications, the containers from this
wait list will be removed.
+
+ 5. Once the size of the waitlist reaches zero; maintenance mode or
decommission is complete.
+
+ 5. We will update the node state to DECOMMISSIONED or IN_MAINTENANCE reached
state.
+
+_Replica count_ is a calculated number which represents the number of
_missing_ replicas. The number can be negative in case of an over-replicated
container.
+
+
+## Calculation of the _Replica count_ (required replicas)
+
+### Counters / Variables
+
+We have 7 different datanode state and three different type of container state
(replicated or in-flight deletion / in-flight replication). To calculate the
required replicas we should introduce a few variables.
+
+Note: we don't need to use all the possible counters but the following table
summarize how the counters are calculated for the following algorithm.
+
+For example the `maintenance` variable includes the number of the existing
replicas on ENTERING_MAINTENANCE or IN_MAINTENANCE nodes.
+
+Each counters should be calculated per container bases.
+
+ Node state | Containers - in-flight deletion |
In-Flight |
+
--------------------------------------|---------------------------------|-------------------------|
+ HEALTHY | `healthy`
| `inFlight`
+ STALE + DEAD + DECOMMISSIONED | |
+ DECOMMISSIONING | |
+ ENTERING_MAINTENANCE + IN_MAINTENANCE | `maintenance` |
+
+### The current replication model
+
+The current replication model in SCM/Ozone is very simplistic. We compute the
replication count or the number of replications that we need to do as:
+
+```
+Replica count = expectedCount - currentCount
+```
+
+In case the _Replica count_ is positive, it means that we need to make more
replicas. If the number is negative, it means that we are over replicated and
we need to remove some replicas of this container. If the Replica count for a
container is zero; it means that we have the expected number of containers in
the cluster.
+
+To support idempontent placement strategies we include the in-fligt
replications in the `currentCount`: If there are one in-flight replication
process and two replicas we won't start a new replication command unless the
original command is timed out.
+
+The timeout is configured with `hdds.scm.replication.event.timeout` and the
default value is 10 minutes.
+
+More preciously the current algorithm is the following:
+
+```
+Replica count = expectedCount - healthy - inFlight
+```
+
+### The proposed solution
+
+To support the notion that a user can provide hints to the replication model,
we propose to add two variables to the current model.
+
+In the new model, we propose to break the `currentCount` into the three
separate groups. That is _Healthy nodes_, _Maintenance nodes_, and
_Decommission nodes_. The new model replaces the currentCount with these three
separate counts. The following function captures the code that drives the logic
of computing Replica counts in the new model. The table below discusses the
input and output of this model very extensively.
+
+```java
+/**
+ * Calculate the number of the missing replicas.
+ *
+ * @return the number of the missing replicas. If it's less than zero, the
container is over replicated.
+ */
+int getReplicationCount(int expectedCount, int healthy,
+ int maintenance, int inFlight) {
+
+ //for over replication, count only with the healthy replicas
+ if (expectedCount < healthy) {
+ return expectedCount - healthy;
+ }
+
+ replicaCount = expectedCount - (healthy + maintenance + inFlight);
+
+ if (replicaCount == 0 && healthy < 1) {
+ replicaCount ++;
+ }
+
+ //over replication is already handled
+ return Math.max(0, replicaCount);
+}
+
+```
+
+We also need to specify two end condition when the DECOMMISSIONING node can be
moved to the DECOMMISSIONED state or the ENTERING_MAINTENANCE mode can be moved
to the IN_MAINTENANCE state.
+
+The following conditions should be true for all the containers and all the
containers on the specific node should be closed.
+
+From DECOMMISSIONING to DECOMMISSIONED:
+
+ * There are at least one healthy replica
+ * There are at most one missing replica
+
+Which means that node can be decommissioned if:
+
+ * all the containers with replication factor THREE have at least *one
replica* on a HEALTHY nodes (minimum.live.replicas)
+ * all the containers with replication factor THREE have at least *three*
replicas on HEALTHY/ENTERING_MAINTENENCE/IN_MAINTENANCE nodes (minimum.replicas)
+ * all the containers with replication factor ONE have on replica on a HEALTHY
node.
+
+
+From ENTERING_MAINTENANCE to IN_MAINTENANCE:
+
+ * There are at least one healthy replicas
+
+Which means that node can be decommissioned if
+
+ * all the containers with replication factor THREE have at least *one
replica* on a HEALTHY nodes (minimum.live.replicas)
+ * all the containers with replication factor ONE have one replica on a
HEALTHY node.
+
+Note: the specified numbers can be cluster-wide configurable.
+
+### Examples (normal cases)
+
+First, let's talk about the simple case where there is no over replication or
in-flight replica copy. In this case the previous
+
+#### All healthy
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | HEALHTY
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 3
+ maintenance | 0
+ replicaCount | 0
+
+The container C1 exists on machines A, B , and C. All the container reports
tell us that the container is healthy. Running the above algorithm, we get:
+
+`expected - healthy + maint. = 3 - (3 + 0) = 0`
+
+It means, _"we don’t need no replication"._
+
+#### One failure
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | DEAD
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 2
+ maintenance | 0
+ replicaCount | 1
+
+
+The machine C has failed, and as a result, the healthy count has gone down
from `3` to `2`. This means that we need to start one replication flow.
+
+`ReplicaCount = expected - healthy + maint. = 3 - (2 + 0) = 1.`
+
+This means that the new model will handle failure cases just like the current
model.
+
+#### One decommissioning
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | DECOMMISSIONING
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 2
+ maintenance | 0
+ replicaCount | 1
+
+
+In this case, machine C is being decommissioned. Therefore the healthy count
has gone down to `2` , and decommission count is `1`. Since the `ReplicaCount =
expected - healthy + maint`. we have `1 = 3 - (2 + 0)`, this gives us the
decommission count implicitly. The trick here is to realize that incrementing
decommission automatically causes a decrement in the healthy count, which
allows us not to have _decommission_ in the equation explicitly.
+
+**Stop condition**: Not that if this containers is the only one on node C,
node C can be moved to the DECOMMISSIONED state.
+
+#### Failure + decommissioning
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | DEAD
+ C | DECOMMISSIONING
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 1
+ maintenance | 0
+ replicaCount | 2
+
+Here is a case where we have a failure of a data node and a decommission of
another data node. In this case, the container C1 needs two replica flows to
heal itself. The equation is the same and we get
+
+`ReplicaCount(2) = ExpectecCount(3) - healthy(1)`
+
+The maintenance is still zero so ignored in this equation.
+
+#### 1 failure + 2 decommissioning
+
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | DECOMMISSIONING
+ C | DECOMMISSIONING
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 0
+ replicaCount | 3
+
+In this case, we have one failed data node and two data nodes being
decommissioned. We need to get three replica flows in the system. This is
achieved by:
+
+```
+ReplicaCount(3) = ExpectedCount(3) - (healthy(0) + maintenance(0))
+```
+
+#### Maintenance mode
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | ENTERING_MAINTENANCE
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 2
+ maintenance | 1
+ replicaCount | 0
+
+This represents the normal maintenance mode, where a single machine is marked
as in maintenance mode. This means the following:
+
+```
+ReplicaCount(0) = ExpectedCount(3) - (healthy(2) + maintenance(1)
+```
+
+There are no replica flows since the user has asked us to move a single node
into maintenance mode, and asked us explicitly not to worry about the single
missing node.
+
+**Stop condition**: Not that if this containers is the only one on node C,
node C can be moved to the IN_MAINTENANCE state.
+
+#### Maintenance + decommissioning
+
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | DECOMMISSIONING
+ C | ENTERING_MAINTENANCE
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 1
+ maintenance | 1
+ replicaCount | 1
+
+*This is a fascinating case*; We have one good node; one decommissioned node
and one node in maintenance mode. The expected result is that the replica
manager will launch one replication flow to compensate for the node that is
being decommissioned, and we also expect that there will be no replication for
the node in maintenance mode.
+
+```
+Replica Count (1) = expectedCount(3) - (healthy(1) + maintenance(1))
+```
+So as expected we have one replication flow in the system.
+
+**Stop condition**: Not that if this containers is the only one in the system:
+
+ * node C can be moved to the IN_MAINTENANCE state
+ * node B can not be decommissioned (we need the three replicas first)
+
+#### Decommissioning all the replicas
+
+ Node with replica | Node status
+ ------------------|------------
+ A | DECOMMISSIONING
+ B | DECOMMISSIONING
+ C | DECOMMISSIONING
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 0
+ replicaCount | 3
+
+In this case, we deal with all the data nodes being decommissioned. The number
of healthy replicas for this container is 0, and hence:
+
+```
+replicaCount (3) = expectedCount (3)- (healthy(0) + maintenance(0)).
+```
+
+This provides us with all 3 independent replica flows in the system.
+
+#### Decommissioning the one remaining replicas
+
+ Node with replica | Node status
+ ------------------|------------
+ A | DEAD
+ B | DEAD
+ C | DECOMMISSIONING
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 0
+ replicaCount | 3
+
+We have two failed nodes and one node in Decomm. It is the opposite of case
Line 5, where we have one failed node and 2 nodes in Decomm. The expected
results are the same, we get 3 flows.
+
+#### Total failure
+
+ Node with replica | Node status
+ ------------------|------------
+ A | DEAD
+ B | DEAD
+ C | DEAD
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 0
+ replicaCount | 3
+
+This is really an error condition. We have lost all 3 data nodes. The Replica
Manager will compute that we need to rebuild 3 replicas, but we might not have
a source to rebuild from.
+
+### Last replica is on ENTERING_MAINTENANCE
+
+ Node with replica | Node status
+ ------------------|------------
+ A | DEAD
+ B | ENTERING_MAINTENANCE
+ C | DEAD
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 1
+ replicaCount | 2
+
+Is also an interesting case; we have lost 2 data nodes; and one node is being
marked as Maint. Since we have 2 failed nodes, we need 2 replica flows in the
system. However, the maintenance mode cannot be entered, since we will lose
lone replica if we do that.
+
+
+### All maintenance
+
+ Node with replica | Node status
+ ------------------|------------
+ A | ENTERING_MAINTENANCE
+ B | ENTERING_MAINTENANCE
+ C | ENTERING_MAINTENANCE
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 3
+ replicaCount | *1*
+
+This is also a very special case; this is the case where the user is telling
us to ignore the peril for all 3 replicas being offline. This means that the
system will not be able to get to that container and would lead to potential
I/O errors. Ozone will strive to avoid that case; this means that Ozone will
hit the “if condition” and discover that we our ReplicCount is 0; since the
user asked for it; but we are also going to lose all Replicas. At this point of
time, we make a conscious decision to replicate one copy instead of obeying the
user command and get to the situation where I/O can fail.
+
+**This brings us back to the semantics of Maintenance mode in Ozone**. If
going into maintenance mode will not lead to a potential I/O failure, we will
enter into the maintenance mode; Otherwise, we will replicate and enter into
the maintenance mode after the replication is done. This is just the core
replication algorithm, not the complete Decommission or Maintenance mode
algorithms, just how the replica manager would behave. Once we define the
behavior of Replica Manager, rest of the algorithm is easy to construct.
+
+### Over replication
+
+For over-replicated containers Ozone prefers to keep the replicas on the
healthy nodes. We delete containers only if we have enough replicas on healthy
nodes.
+
+```
+ //for over replication, count only with the healthy replicas
+ if (expectedCount < healthy) {
+ return expectedCount - healthy;
+ }
+```
+
+Please note that we always assume that the the in-flight deletion are applied
and the container is already deleted.
+
+There is a very rare case where the in-flight deletion is timed out (and as a
result replication manager would assume the container is not deleted) BUT in
the mean-time the container finally deleted. It can be survivied with including
the creation timestamp in the ContainerDeleteCommand.
+
+### Over replication examples
+
+#### 4 replicas
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | HEALTHY
+ D | HEALTHY
+
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 4
+ maintenance | 0
+ replicaCount | -1
+
+This is an easy cas as we have too many replicas we can safely remove on.
+
+#### over replicated with IN_MAINTENANCE
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | HEALTHY
+ D | IN_MAINTENANCE
+
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 3
+ maintenance | 1
+ replicaCount | 0
+
+In this case we will delete the forth replica only after node D is restored
and healthy again. (expectedCount is not less than healthy)
+
+#### over replicated with IN_MAINTENANCE
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | IN_MAINTENANCE
+ D | IN_MAINTENANCE
+
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 2
+ maintenance | 2
+ replicaCount | 0
+
+Here we are not over-repliacated as we don't have any healthy nodes.
+
+The main algorithm would return with `replicaCount = -1` but as we return
`Math.max(0,replicaCount)` the real response will be 0. Waiting for healthy
nodes.
+
+### Handling in-flight replications
+
+Let's say we have an under-replicated container and we already selected a new
datanode to copy a new replica to that specific node.
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | HEALTHY
+ C | (in-flight)
+
+In this case we have only two replicas one replica is missing
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 2
+ maintenance | 0
+ replicaCount | 1 (without calculating with in-flight)
+
+Here the Replication Manager detects that one replica is missing but we
wouldn't like to select a new datanode based on the ContainerPlacementPolicy
implementation as the policy may or may not be idempotent.
+
+For example if the placement policy would select a datanode randomly with each
loop we would select a new datanode to replicate to.
+
+To avoid such a situation we maintain a list of the in-flight copies
(in-memory) on the SCM side. In this list we have all the sent replication
requests but they are removed after a given amount of time (10 minutes) by
default.
+
+With calculating the in-flight copy as a possible replication the Replication
Manger doesn't need to request new replication.
+
+The algorithm can be supported with decommissioning / maintenance mode:
+
+When a datanode is marked to be decommissioned there could be any in-flight
replication copy process in that time.
+
+ * At datanode we should stop all of the in-flight copy (datanodes should be
notified about the DECOMMISSIONING/IN_MAINTENANCE state)
+ * We never ask any non-healthy nodes to replicate containers.
+ * In SCM, we don't need to do any special action
+ * In `ReplicationManager` we already have a map about the inflight
replications (`Map<ContainerID, List<InflightAction>>`).
+ * During a normal replication the number of in-flight replications are
counted as real replication (2 real replicas + 1 inflight replication = replica
count 3). During this calculation we need to check the current state of the
datanodes and ignore the inflight replication if they are assigned to a node
which is in decommissioning state. (Or we should update the inflight map, in
case of node state change)
+
+### In-flight examples
+
+#### Maintenance + inflight
+
+ Node with replica | Node status
+ ------------------|------------
+ A | HEALTHY
+ B | ENTERING_MAINTENANCE
+ C | (missing) + inflight
+
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 1
+ maintenance | 1
+ in-flight | 1
+ replicaCount | 0
+
+Here one have one node ENTERING_MAINTENANCE state, and one replica is missing
and already started to be replicated. We don't need to start a new copy and
node B can be moved to the IN_MAINTENANCE mode.
+
+```
+Replica Count (0) = expectedCount(3) - (healthy(1) + maintenance(1) -
inFlihgt(1))
+```
+
+
+#### Maintenance + inflight
+
+ Node with replica | Node status
+ ------------------|------------
+ A | DECOMMISSIONING
+ B | (missing) + inflight
+ C | (missing) + inflight
+
+
+ Counter | Value
+ ------------------ | -------------
+ expectedCount | 3
+ healthy | 0
+ maintenance | 0
+ in-flight | 2
+ replicaCount | 1
+
+We have on emissing replicas, but and node A can not be DECOMMISSIONED as we
have no HEALTHY replicas at all.
+
+
+## Statefulness
+
+SCM stores all the node state in-memory. After a restart on the SCM side the
datanode state can be lost.
+
+**Ozone doesn't guarantee that decommissioning/maintenance mode state survives
the SCM restarts!!!**
+
+ * If SCM restarts DECOMMISSIONED nodes will not report any more container
reports and the nodes won't be registered.
+ * ENTERING_MAINTENANCE and DECOMMISSIONING nodes will became HEALTHY again
and the decommissioning CLI command should be repeated.
+ * IN_MAINTENANCE nodes will become DEAD and all the containers will be
replicated.
+
+ *Ozone assumes that the maintenance mode is used short-term and SCM is not
restarted during this specific period.*
+
+
+*Reasoning*:
+
+Neither of the node state nor the container state are persisted in SCM side.
The decommissioned state can be stored on the SCM side (or on the SCM side and
the datanode side) which can provide better user experience (and may be
implemented).
+
+But to support maintenance mode after restart all the container information is
required to be persisted (which is a too big architectural change).
+
+To make a replication decision replication manager needs the number of healthy
replicas (they are reported via heartbeats) AND the number of containers on the
node which is in maintenance mode. The later one is not available if the SCM is
restarted as the container map exists only in the memory and the node which is
turned off can't report any more container reports. Therefore the information
about the existing containers on the node which is in the maintenance mode
**can't be available'**.
+
+## Throttling
+
+SCM should avoid to request too many replication to live enough network
bandwidth for the requests.
+
+Replication Manager can easily throttle the replication requests based on
`inflightReplication` map.
+
+The first algorithm would specify the maximum number of the requests per node
which should be checked before sendint out the replication request to the
datanode.
+
+With the same approach more advanced calculation can be easily implemented
(cluster-wide limit, topology-related limits, etc.)
+
+## User interface
+
+The decommissioning and maintenance mode can be administered with a CLI
interface.
+
+Required feature:
+
+ * Set the state of a datanode (to DECOMMISSIONING or ENTERING_MAINTENANCE)
+ * Undo the decommissioning process
+ * check the current progress:
+ * This can be a table with the nodes, status of the nodes, number of
containers, containers under replication and containers which doesn't much the
stop condition yet (required replications)
+ * All the commands can support topology related filters (eg. display the
nodes only for a specific rack or show the status of the nodes of s specific
rack)
+
+## Checks before the decommissioning
+
+Decommissioning is requested via a new RPC call with the help of a new CLI
tool. The server should check the current state of the cluster and deny the
decommissioning if it's not possible. Possible violations:
+
+ * Not enough space to store the new replicas.
+ * Not enough node to create all kind of pipelines
+
+ In case of any violation, the request will fail, but any of theses rules can
be turned off with a next request and the decommissioning can be forced.
+
+## Maintain progress
+
+We need to show the progress of the decommissioning process per node and
cluster-wide. We already have the information about the under replicated
containers, but we don't know the numbers of the containers before
decommissioning.
+
+Instead of saving the original number of the required replications before
(which is very fragile) we don't provide an absolute progress just the numbers
of the remaining replication:
+
+ Node | Status | # containers | in-progress
replications | required replication
Review comment:
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