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https://issues.apache.org/jira/browse/ZOOKEEPER-2619?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15605104#comment-15605104
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Flavio Junqueira edited comment on ZOOKEEPER-2619 at 10/25/16 12:01 PM:
------------------------------------------------------------------------
bq. In my three-line example, it doesn't matter whether the second create is
synchronous (mixing) or asynchronous (not mixing). Isn't it fine to issue a
series of asynchronous calls ending with a synchronous call?
Maybe, it depends on where you make the synchronous call. If you call it from a
callback, then you may end up re-ordering the results. After checking it again,
I don't think it is super important for this discussion. In your simple
example, you're right that it doesn't matter whether the second call is sync or
async.
I like the idea of not making the behavior global and configurable via switch.
In particular, the configuration switch is error prone. I still think that
mixing API use and different behavior, even across modules, with the same ZK
handle isn't good practice, but I'm fine with leaving it up to the application
to decide, though.
On {{getConnection}, I have a few of high-level points:
# I like the fact that it is all client side and it makes it clear to the
application that requests are being sent over a given connection. Once the
connection drops, a proper suffix of the operations will receive an error.
# I'd rather not have the sync api and the async api over different objects.
The proposal has the sync api in the zookeeper handle and the async api in the
connection object. I'll discuss this some more below.
# I don't like much the idea of exposing a "connection". The abstraction we
have historically is a session and the fact that a session can have multiple
connections over time is mostly transparent to the application, except for the
connection loss event, which makes it clear that there are multiple connections
going on underneath.
On the second point, we don't necessarily have to move the async api to such a
connection object. We could instead have something like a token and the token
essentially represents a connection. If the connection drops, then the token is
invalidated. With such a token, we will be adding a new call for each
operations to the handle, something like:
{code}
public void create(final String path,
byte data[],
List<ACL> acl,
CreateMode createMode,
StringCallback cb,
Object ctx,
FIFOToken token);
{code}
and also a call to get the current token:
{code}
public FIFOToken getFifoToken() {
return this.fifoToken;
}
{code}
Again, the main reason for suggesting this is to enable such FIFO-enforced
calls in the handle API rather than having a connection object.
Another point I was thinking about with respect to the use of {{getConnection}}
is the following. If I'm a developer, where should I call it and when? In
principle, we can do it upon receiving a {{SyncConnected}} event via the
default watcher (transition from {{Disconnected}} to {{SyncConnected}}. It
might not be possible to do it at that point in the case I have one or more
threads submitting operations, though. The applications will need to pause
before we get the new connection, otherwise we can have a mix of threads
sending to the invalid connection while others send to the new connection. In
the case it is ok that the FIFO order guarantee is per thread, we can have each
thread individually calling {{getConnection}} (or whatever other variant we end
up agreeing upon) upon receiving a connection loss error.
The last point of the list is important because we have proposed quite some
time back to remove connection loss by resending upon reconnecting in
ZOOKEEPER-22. If we resend, then we won't be erroring out requests with a
connection loss event upon dropping a connection. If we do it, then we avoid
the gaps that we are discussing in this issue in the first place. ZOOKEEPER-22
needs work on the server-side, though. On the positive side, the client-side
API changes are minimal.
was (Author: fpj):
bq. In my three-line example, it doesn't matter whether the second create is
synchronous (mixing) or asynchronous (not mixing). Isn't it fine to issue a
series of asynchronous calls ending with a synchronous call?
Maybe, it depends on where you make the synchronous call. If you call it from a
callback, then you may end up re-ordering the results. After checking it again,
I don't think it is super important for this discussion. In your simple
example, you're right that it doesn't matter whether the second call is sync or
async.
I like the idea of not making the behavior global and configurable via switch.
In particular, the configuration switch is error prone. I still think that
mixing API use and different behavior, even across modules, with the same ZK
handle isn't good practice, but I'm fine with leaving it up to the application
to decide, though.
On {{getConnection}, I have a few of high-level points:
# I like the fact that it is all client side and it makes it clear to the
application that requests are being sent over a given connection. Once the
connection drops, a proper suffix of the operations will receive an error.
# I'd rather not have the sync api and the async api over different objects.
The proposal has the sync api in the zookeeper handle and the async api in the
connection object. I'll discuss this some more below.
# I don't like much the idea of exposing a "connection". The abstraction we
have historically is a session and the fact that a session can have multiple
connections over time is mostly transparent to the application, except for the
connection loss event, which makes it clear that there are multiple connections
going on underneath.
On the second point, we don't necessarily have to move the async api to such a
connection object. We could instead have something like a token and the token
essentially represents a connection. If the connection drops, then the token is
invalidated. With such a token, we will be adding a new call for each
operations to the handle, something like:
{code}
public void create(final String path,
byte data[],
List<ACL> acl,
CreateMode createMode,
StringCallback cb,
Object ctx,
FIFOToken token);
{code}
and also a call to get the current token:
{code}
public FIFOToken getFifoToken() {
return this.fifoToken;
}
{code}
Again, the main reason for suggesting this is to enable such FIFO-enforced
calls in the handle API rather than having a connection object.
Another point I was thinking about with respect to the use of {{getConnection}}
is the following. If I'm a developer, where should I call it and when? In
principle, we can do it upon receiving a {{SyncConnected}} event via the
default watcher (transition from {{Disconnected}} to {{SyncConnected}). It
might not be possible to do it at that point in the case I have one or more
threads submitting operations, though. The applications will need to pause
before we get the new connection, otherwise we can have a mix of threads
sending to the invalid connection while others send to the new connection. In
the case it is ok that the FIFO order guarantee is per thread, we can have each
thread individually calling {{getConnection}} (or whatever other variant we end
up agreeing upon) upon receiving a connection loss error.
The last point of the list is important because we have proposed quite some
time back to remove connection loss by resending upon reconnecting in
ZOOKEEPER-22. If we resend, then we won't be erroring out requests with a
connection loss event upon dropping a connection. If we do it, then we avoid
the gaps that we are discussing in this issue in the first place. ZOOKEEPER-22
needs work on the server-side, though. On the positive side, the client-side
API changes are minimal.
> Client library reconnecting breaks FIFO client order
> ----------------------------------------------------
>
> Key: ZOOKEEPER-2619
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2619
> Project: ZooKeeper
> Issue Type: Bug
> Reporter: Diego Ongaro
>
> According to the USENIX ATC 2010
> [paper|https://www.usenix.org/conference/usenix-atc-10/zookeeper-wait-free-coordination-internet-scale-systems],
> ZooKeeper provides "FIFO client order: all requests from a given client are
> executed in the order that they were sent by the client." I believe
> applications written using the Java client library are unable to rely on this
> guarantee, and any current application that does so is broken. Other client
> libraries are also likely to be affected.
> Consider this application, which is simplified from the algorithm described
> on Page 4 (right column) of the paper:
> {code}
> zk = new ZooKeeper(...)
> zk.createAsync("/data-23857", "...", callback)
> zk.createSync("/pointer", "/data-23857")
> {code}
> Assume an empty ZooKeeper database to begin with and no other writers.
> Applying the above definition, if the ZooKeeper database contains /pointer,
> it must also contain /data-23857.
> Now consider this series of unfortunate events:
> {code}
> zk = new ZooKeeper(...)
> // The library establishes a TCP connection.
> zk.createAsync("/data-23857", "...", callback)
> // The library/kernel closes the TCP connection because it times out, and
> // the create of /data-23857 is doomed to fail with ConnectionLoss. Suppose
> // that it never reaches the server.
> // The library establishes a new TCP connection.
> zk.createSync("/pointer", "/data-23857")
> // The create of /pointer succeeds.
> {code}
> That's the problem: subsequent operations get assigned to the new connection
> and succeed, while earlier operations fail.
> In general, I believe it's impossible to have a system with the following
> three properties:
> # FIFO client order for asynchronous operations,
> # Failing operations when connections are lost, AND
> # Transparently reconnecting when connections are lost.
> To argue this, consider an application that issues a series of pipelined
> operations, then upon noticing a connection loss, issues a series of recovery
> operations, repeating the recovery procedure as necessary. If a pipelined
> operation fails, all subsequent operations in the pipeline must also fail.
> Yet the client must also carry on eventually: the recovery operations cannot
> be trivially failed forever. Unfortunately, the client library does not know
> where the pipelined operations end and the recovery operations begin. At the
> time of a connection loss, subsequent pipelined operations may or may not be
> queued in the library; others might be upcoming in the application thread. If
> the library re-establishes a connection too early, it will send pipelined
> operations out of FIFO client order.
> I considered a possible workaround of having the client diligently check its
> callbacks and watchers for connection loss events, and do its best to stop
> the subsequent pipelined operations at the first sign of a connection loss.
> In addition to being a large burden for the application, this does not solve
> the problem all the time. In particular, if the callback thread is delayed
> significantly (as can happen due to excessive computation or scheduling
> hiccups), the application may not learn about the connection loss event until
> after the connection has been re-established and after dependent pipelined
> operations have already been transmitted over the new connection.
> I suggest the following API changes to fix the problem:
> - Add a method ZooKeeper.getConnection() returning a ZKConnection object.
> ZKConnection would wrap a TCP connection. It would include all synchronous
> and asynchronous operations currently defined on the ZooKeeper class. Upon a
> connection loss on a ZKConnection, all subsequent operations on the same
> ZKConnection would return a Connection Loss error. Upon noticing, the client
> would need to call ZooKeeper.getConnection() again to get a working
> ZKConnection object, and it would execute its recovery procedure on this new
> connection.
> - Deprecate all asynchronous methods on the ZooKeeper object. These are
> unsafe to use if the caller assumes they're getting FIFO client order.
> - No changes to the protocols or servers are required.
> I recognize this could cause a lot of code churn for both ZooKeeper and
> projects that use it. On the other hand, the existing asynchronous calls in
> applications should now be audited anyhow.
> The code affected by this issue may be difficult to contain:
> - It likely affects all ZooKeeper client libraries that provide both
> asynchronous operations and transparent reconnection. That's probably all
> versions of the official Java client library, as well as most other client
> libraries.
> - It affects all applications using those libraries that depend on the FIFO
> client order of asynchronous operations. I don't know how common that is, but
> the paper implies that FIFO client order is important.
> - Fortunately, the issue can only manifest itself when connections are lost
> and transparently reestablished. In practice, it may also require a long
> pipeline or a significant delay in the application thread while the library
> establishes a new connection.
> - In case you're wondering, this issue occurred to me while working on a new
> client library for Go. I haven't seen this issue in the wild, but I was able
> to produce it locally by placing sleep statements in a Java program and
> closing its TCP connections.
> I'm new to this community, so I'm looking forward to the discussion. Let me
> know if I can clarify any of the above.
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