117 lines
5.1 KiB
Markdown
117 lines
5.1 KiB
Markdown
# Client Reconnection and Session Reattachment
|
|
|
|
Apache ActiveMQ Artemis clients can be configured to automatically reconnect or
|
|
re-attach to the server in the event that a failure is detected in the
|
|
connection between the client and the server.
|
|
|
|
## 100% Transparent session re-attachment
|
|
|
|
If the failure was due to some transient failure such as a temporary
|
|
network failure, and the target server was not restarted, then the
|
|
sessions will still be existent on the server, assuming the client
|
|
hasn't been disconnected for more than connection-ttl [Detecting Dead Connections](connection-ttl.md)
|
|
|
|
In this scenario, Apache ActiveMQ Artemis will automatically re-attach the client
|
|
sessions to the server sessions when the connection reconnects. This is
|
|
done 100% transparently and the client can continue exactly as if
|
|
nothing had happened.
|
|
|
|
The way this works is as follows:
|
|
|
|
As Apache ActiveMQ Artemis clients send commands to their servers they store each sent
|
|
command in an in-memory buffer. In the case that connection failure
|
|
occurs and the client subsequently reattaches to the same server, as
|
|
part of the reattachment protocol the server informs the client during
|
|
reattachment with the id of the last command it successfully received
|
|
from that client.
|
|
|
|
If the client has sent more commands than were received before failover
|
|
it can replay any sent commands from its buffer so that the client and
|
|
server can reconcile their states.Ac
|
|
|
|
The size of this buffer is configured with the `confirmationWindowSize`
|
|
parameter on the connection URL. When the server has received
|
|
`confirmationWindowSize` bytes of commands and processed them it will
|
|
send back a command confirmation to the client, and the client can then
|
|
free up space in the buffer.
|
|
|
|
The window is specified in bytes.
|
|
|
|
Setting this parameter to `-1` disables any buffering and prevents any
|
|
re-attachment from occurring, forcing reconnect instead. The default
|
|
value for this parameter is `-1`. (Which means by default no auto
|
|
re-attachment will occur)
|
|
|
|
## Session reconnection
|
|
|
|
Alternatively, the server might have actually been restarted after
|
|
crashing or being stopped. In this case any sessions will no longer be
|
|
existent on the server and it won't be possible to 100% transparently
|
|
re-attach to them.
|
|
|
|
In this case, Apache ActiveMQ Artemis will automatically reconnect the connection and
|
|
*recreate* any sessions and consumers on the server corresponding to the
|
|
sessions and consumers on the client. This process is exactly the same
|
|
as what happens during failover onto a backup server.
|
|
|
|
Client reconnection is also used internally by components such as core
|
|
bridges to allow them to reconnect to their target servers.
|
|
|
|
Please see the section on failover [Automatic Client Failover](ha.md) to get a full understanding of how
|
|
transacted and non-transacted sessions are reconnected during
|
|
failover/reconnect and what you need to do to maintain *once and only
|
|
once*delivery guarantees.
|
|
|
|
## Configuring reconnection/reattachment attributes
|
|
|
|
Client reconnection is configured using the following parameters:
|
|
|
|
- `retryInterval`. This optional parameter determines the period in
|
|
milliseconds between subsequent reconnection attempts, if the
|
|
connection to the target server has failed. The default value is
|
|
`2000` milliseconds.
|
|
|
|
- `retryIntervalMultiplier`. This optional parameter determines
|
|
determines a multiplier to apply to the time since the last retry to
|
|
compute the time to the next retry.
|
|
|
|
This allows you to implement an *exponential backoff* between retry
|
|
attempts.
|
|
|
|
Let's take an example:
|
|
|
|
If we set `retryInterval` to `1000` ms and we set
|
|
`retryIntervalMultiplier` to `2.0`, then, if the first reconnect
|
|
attempt fails, we will wait `1000` ms then `2000` ms then `4000` ms
|
|
between subsequent reconnection attempts.
|
|
|
|
The default value is `1.0` meaning each reconnect attempt is spaced
|
|
at equal intervals.
|
|
|
|
- `maxRetryInterval`. This optional parameter determines the maximum
|
|
retry interval that will be used. When setting
|
|
`retryIntervalMultiplier` it would otherwise be possible that
|
|
subsequent retries exponentially increase to ridiculously large
|
|
values. By setting this parameter you can set an upper limit on that
|
|
value. The default value is `2000` milliseconds.
|
|
|
|
- `reconnectAttempts`. This optional parameter determines the total
|
|
number of reconnect attempts to make before giving up and shutting
|
|
down. A value of `-1` signifies an unlimited number of attempts. The
|
|
default value is `0`.
|
|
|
|
All of these parameters are set on the URL used to connect to the broker.
|
|
|
|
If your client does manage to reconnect but the session is no longer
|
|
available on the server, for instance if the server has been restarted
|
|
or it has timed out, then the client won't be able to re-attach, and any
|
|
`ExceptionListener` or `FailureListener` instances registered on the
|
|
connection or session will be called.
|
|
|
|
ExceptionListeners and SessionFailureListeners
|
|
==============================================
|
|
|
|
Please note, that when a client reconnects or re-attaches, any
|
|
registered JMS `ExceptionListener` or core API `SessionFailureListener`
|
|
will be called.
|