Map Configuration

In Hazelcast, you can create custom map configurations associated with the names of maps. The maps that do not have any configuration defined use the default configuration. If you want to set a configuration that is valid for all maps, you can name your configuration as default. A user-defined default configuration applies to every map that does not have a specific custom map configuration defined with the map’s name. You can also use wildcards to associate your configuration with multiple map names. See the configuration documentation for more information about wildcards.

Hazelcast distributes the map entries onto multiple cluster members. Each member holds some portion of the data. Distributed maps have one backup by default. If a member goes down, your data is recovered using the backups in the cluster. You can set the backup count to 0, 1, or 2 depending on your requirements.

Time To Live (TTL) is the maximum time in seconds for each entry to stay in the map. It limits the lifetime of the entries relative to the time of the last write access performed on them. If it is not 0, the entries whose lifetime exceeds this period (without any write access performed on them during this period) are expired and evicted automatically. An individual entry may have its own lifetime limit by using one of the methods accepting a TTL. See Evicting Specific Entries section in the Hazelcast map documentation. If there is no TTL value provided for the individual entry, it inherits the value set for this element. Valid values are integers between 0 and Integer.MAX VALUE. Its default value is 0, which means infinite (no expiration and eviction). If it is not 0, entries are evicted regardless of the set eviction policy described below.

Maximum time in seconds for each entry to stay idle in the map. It limits the lifetime of the entries relative to the time of the last read or write access performed on them. The entries whose idle period exceeds this limit are expired and evicted automatically. An entry is idle if no get, put or containsKey is called on it. Valid values are integers between 0 and Integer.MAX_VALUE. Its default value is 0, which means infinite.

Both Time-to-Live and Max Idle Seconds can be used simultaneously on the map entries. In that case, the entry is considered expired if at least one of the policies marks it as expired.

Eviction policy to be applied when the size of the map grows larger than the value specified by the Max Size element described below. Valid values are as follows:

Policy on how to define the map’s maximum size. Hazelcast detects if a map has reached its maximum size based on the Max Size Policy and then executes the eviction. The options are as follows:

The percentage value when the eviction will start depending on the selected Max Size Policy.

You can add indexes to your map. Hazelcast distributed queries run on each member in parallel and return only the results to the caller. Then, on the caller side, the results are merged.

When a query runs on a member, Hazelcast iterates through all the owned entries and finds the matching ones. This can be made faster by indexing the most frequently queried fields, just like you would do for your database. Indexing adds an overhead for each write operation but the queries will be a lot faster. If you query your map a lot, make sure to add indexes for the most frequently queried fields. For example, if you do an active and age < 30 query, make sure you add an index for the active and age fields.