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https://issues.apache.org/jira/browse/HBASE-2294?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12845665#action_12845665
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Todd Lipcon commented on HBASE-2294:
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Here's a first pass at some kind of spec. These aren't meant to be final - just
posting for discussion. I anticipate that after we (developers) come to some
kind of conclusion here we will want to run this by the user list to see if
we're missing use cases, etc.
h1. Definitions
For the sake of common vocabulary, we define the following terms:
*ATOMICITY*: an operation is atomic if it either completes entirely or not at
all
*CONSISTENCY*: all actions cause the table to transition from one valid state
directly to another (eg a row will not disappear during an update,e tc)
*ISOLATION*: an operation is isolated if it appears to complete independently
of any other concurrent transaction
*DURABILITY*: any update that reports "successful" to the client will not be
lost
*VISIBILITY*: an update is considered visible if any subsequent read will see
the update as having been committed
h1. APIs to consider
* Read APIs
** get
** scan
* Write APIs
** put
** delete
* Combination (read-modify-write) APIs
** incrementColumnValue
** compareAndSet
h1. Guarantees Provided
h2. Atomicity
# All mutations are atomic within a row. Any put will either wholely succeed or
wholely fail.
## An operation that returns a "success" code has completely succeeded.
## An operation that returns a "failure" code has completely failed.
## An operation that times out may have succeeded and may have failed. However,
it will not have partially succeeded or failed.
# This is true even if the mutation crosses multiple column families within a
row.
# APIs that mutate several rows will _not_ be atomic across the multiple rows.
For example, a multiput that operates on rows 'a','b', and 'c' may return
having mutated some but not all of the rows. XXX: will they return failure or
success or some mixed response here?
# The compareAndSet API happens atomically as is typically understood by this
operation.
h2. Consistency and Isolation
# All rows returned via any access API will consist of a complete row that
existed at some point in the table's history.
# This is true across column families - i.e a get of a full row that occurs
concurrent with some mutations 1,2,3,4,5 will return a complete row that
existed at some point in time between mutation i and i+1 for some i between 1
and 5.
h3. Consistency of Scans
A scan is *not* a consistent view of a table. Scans do *not* exhibit _snapshot
isolation_.
Rather, scans have the following properties:
# Any row returned by the scan will be a consistent view (i.e. that version of
the complete row existed at some point in time)
# A scan will always reflect a version _at least as new as_ the beginning of
the scan. This satisfies the visibility guarantees enumerated below.
## For example, if client A writes data X and then communicates via a side
channel to client B, any scans started by client B will contain data at least
as new as X.
## Scans may include data that is _newer_ than the start of the scan.
## Another way of stating this is that a scan must reflect all mutations
committed prior to the construction of the scanner, and _may_ reflect some
mutations committed subsequent to the construction of the scanner.
Those familiar with relational databases will recognize this isolation level as
"read committed".
XXX: Ryan has mentioned the model of "scans will always get the most up-to-date
version of a row when beginning a new row". Do we want to guarantee this or
just leave it at "some version of the row at least as new as what existed at
scan start"?
h2. Visibility
# When a client receives a "success" response for any mutation, that mutation
is immediately visible to both that client and any client with whom it later
communicates through side channels.
# A row will never exhibit so-called "time-travel" properties. That is to say,
if a series of mutations moves a row sequentially through a series of states,
any sequence of concurrent reads will return a subsequence of those states.
## For example, if a row's cells are mutated using the "incrementColumnValue"
API, a client will never see the value of any cell decrease.
## This is true regardless of which read API is used to read back the mutation.
# Any version of a cell that has been returned to a read operation is
guaranteed to be durably stored.
h2. Durability
# All visible data is also durable data. That is to say, a read will never
return data that is not durably on disk.
# Any operation that returns a "success" code (eg does not throw an exception)
will be made durable.
# Any operation that returns a "failure" code will not be made durable (subject
to the Atomicity guarantees above)
# All reasonable failure scenarios will not affect any of the guarantees of
this document.
XXX: should expand this to include the concept of tunable durability windows
(this also impacts visibility since you can experience time travel during
failure if some updates arent durable)
> Enumerate ACID properties of HBase in a well defined spec
> ---------------------------------------------------------
>
> Key: HBASE-2294
> URL: https://issues.apache.org/jira/browse/HBASE-2294
> Project: Hadoop HBase
> Issue Type: Task
> Components: documentation
> Reporter: Todd Lipcon
> Priority: Blocker
> Fix For: 0.20.4, 0.21.0
>
>
> It's not written down anywhere what the guarantees are for each operation in
> HBase with regard to the various ACID properties. I think the developers know
> the answers to these questions, but we need a clear spec for people building
> systems on top of HBase. Here are a few sample questions we should endeavor
> to answer:
> - For a multicell put within a CF, is the update made durable atomically?
> - For a put across CFs, is the update made durable atomically?
> - Can a read see a row that hasn't been sync()ed to the HLog?
> - What isolation do scanners have? Somewhere between snapshot isolation and
> no isolation?
> - After a client receives a "success" for a write operation, is that
> operation guaranteed to be visible to all other clients?
> etc
> I see this JIRA as having several points of discussion:
> - Evaluation of what the current state of affairs is
> - Evaluate whether we currently provide any guarantees that aren't useful to
> users of the system (perhaps we can drop in exchange for performance)
> - Evaluate whether we are missing any guarantees that would be useful to
> users of the system
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