Hazelcast uses operations to make remote calls. For example, a
map.get is an operation and a
map.put is one operation for the primary
and one operation for each of the backups, i.e.
map.put is executed for the primary and also for each backup. In most cases, there will be a natural balance between the number of threads performing operations
and the number of operations being executed. However, there are two situations where this balance and operations
can pile up and eventually lead to Out of Memory Exception (OOME):
Asynchronous calls: With async calls, the system may be flooded with the requests.
Asynchronous backups: The asynchronous backups may be piling up.
To prevent the system from crashing, Hazelcast provides back pressure. Back pressure works by:
limiting the number of concurrent operation invocations,
periodically making an async backup sync.
Back pressure is disabled by default and you can enable it using the following system property:
To control the number of concurrent invocations, you can configure the number of invocations allowed per partition using the following system property:
The default value of this system property is 100. Using a default configuration a system is allowed to have (271 + 1) * 100 = 27200 concurrent invocations (271 partitions + 1 for generic operations).
Back pressure is only applied to normal operations. System operations like heart beats and partition migration operations
are not influenced by back pressure. 27200 invocations might seem like a lot, but keep in mind that executing a task on
or acquiring a lock also requires an operation.
If the maximum number of invocations has been reached, Hazelcast will automatically apply an exponential back off policy. This
gives the system some time to deal with the load. Using the following system property, you can configure the maximum time to wait before a
HazelcastOverloadException is thrown:
This system property's default value is 60000 ms.
The Health Monitor keeps an eye on the usage of the invocations. If it sees a member has consumed 70% or more of the invocations, it starts to log health messages.
Apart from controlling the number of invocations, you also need to control the number of pending async backups. This is done by periodically making these backups sync instead of async. This forces all pending backups to get drained. For this, Hazelcast tracks the number of asynchronous backups for each partition. At every Nth call, one synchronization is forced. This N is controlled through the following property:
This system property's default value is 100. It means, out of 100 asynchronous backups, Hazelcast makes 1 of them a synchronous one. A randomization is added, so the sync window with default configuration will be between 75 and 125 invocations.
To prevent the system at the client side from overloading, you can apply a constraint on the number of concurrent invocations. You can use the following system property at the client side for this purpose:
This property defines the maximum allowed number of concurrent invocations. When it is not explicitly set, it has the value
Integer.MAX_VALUE by default, which means infinite. When you set it and if the maximum number of concurrent invocations is exceeded this value, Hazelcast will throw
HazelcastOverloadException when a new invocation comes in.
Please note that back off timeout and controlling the number of pending async backups (sync window) is not supported at the client side.
Please refer to the System Properties section to learn how to configure the system properties.