|Events and Event Handling / Configuring Client/Server Event Messaging|
Use durable messaging for subscriptions that you need maintained for your clients even when your clients are down or disconnected. You can configure any of your continuous queries or event subscriptions as durable. Events for durable queries and subscriptions are saved in queue when the client is disconnected and played back when the client reconnects. Other queries and subscriptions are removed from the queue.
Use durable messaging for client/server installations that use continuous queries or event subscriptions.
Properties props = new Properties(); props.setProperty("durable-client-id", "31"); props.setProperty("durable-client-timeout", "" + 200); DistributedSystem ds = DistributedSystem.connect(props);
The durable-client-id indicates that the client is durable and gives the server an identifier to correlate the client to its durable messages. For a non-durable client, this id is an empty string. The ID can be any number that is unique among the clients attached to servers in the same distributed system.
The durable-client-timeout tells the server how long to wait for client reconnect. When this timeout is reached, the server stops storing to the client's message queue and discards any stored messages. The default is 300 seconds. This is a tuning parameter. If you change it, take into account the normal activity of your application, the average size of your messages, and the level of risk you can handle, both in lost messages and in the servers' capacity to store enqueued messages. Assuming that no messages are being removed from the queue, how long can the server run before the queue reaches the maximum capacity? How many durable clients can the server handle? To assist with tuning, use the GemFire message queue statistics for durable clients through the disconnect and reconnect cycles.
The register interest and query creation methods all have an optional boolean parameter for indicating durability. By default all are non-durable.
// Durable registration // Define keySpecification, interestResultPolicy, durability exampleRegion.registerInterest(keySpecification, interestResultPolicySpecification, true); // Durable CQ // Define cqName, queryString, cqAttributes, durability CqQuery myCq = queryService.newCq(cqName, queryString, cqAttributes, true);
Save only critical messages while the client is disconnected by only indicating durability for critical subscriptions and CQs. When the client is connected to its servers, it receives messages for all keys and queries reqistered. When the client is disconnected, non-durable interest registrations and CQs are discontinued but all messages already in the queue for them remain there.
ClientCache clientCache = ClientCacheFactory.create(); // Here, create regions, listeners, and CQs that are not defined in the cache.xml . . . // Here, run all register interest calls before doing anything else clientCache.readyForEvents();