Chapter 9. State Transfer
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## Chapter 9. State Transfer

"State Transfer" refers to the process by which a JBoss Cache instance prepares itself to begin providing a service by acquiring the current state from another cache instance and integrating that state into its own state.

## 9.1. Types of State Transfer

The state that is acquired and integrated can consist of two basic types:

1. "Transient" or "in-memory" state. This consists of the actual in-memory state of another cache instance -- the contents of the various in-memory nodes in the cache that is providing state are serialized and transferred; the recipient deserializes the data, creates corresponding nodes in its own in-memory tree, and populates them with the transferred data.

   "In-memory" state transfer is enabled by setting the cache's `FetchInMemoryState` property to `true`.

2. "Persistent" state. Only applicable if a non-shared cache loader is used. The state stored in the state-provider cache's persistent store is deserialized and transferred; the recipient passes the data to its own cache loader, which persists it to the recipient's persistent store.

   "Persistent" state transfer is enabled by setting a cache loader's `CacheLoaderFetchPersistentState` property to `true`. If multiple cache loaders are configured in a chain, only one can have this property set to true; otherwise you will get an exception at startup.

   Persistent state transfer with a shared cache loader does not make sense, as the same persistent store that provides the data will just end up receiving it. Therefore, if a shared cache loader is used, the cache will not allow a persistent state transfer even if a cache loader has `CacheLoaderFetchPersistentState` set to `true`.

Which of these types of state transfer is appropriate depends on the usage of the cache.

1. If a write-through cache loader is used, the current cache state is fully represented by the persistent state. Data may have been evicted from the in-memory state, but it will still be in the persistent store. In this case, if the cache loader is not shared, persistent state transfer is used to ensure the new cache has the correct state. In-memory state can be transferred as well if the desire is to have a "hot" cache -- one that has all relevant data in memory when the cache begins providing service. (Note that the "CacheLoaderPreload" configuration parameter can be used as well to provide a "warm" or "hot" cache without requiring an in-memory state transfer. This approach somewhat reduces the burden on the cache instance providing state, but increases the load on the persistent store on the recipient side.)

2. If a cache loader is used with passivation, the full representation of the state can only be obtained by combining the in-memory (i.e. non-passivated) and persistent (i.e. passivated) states. Therefore an in-memory state transfer is necesssary. A persistent state transfer is necessary if the cache loader is not shared.

3. If no cache loader is used and the cache is solely a write-aside cache (i.e. one that is used to cache data that can also be found in a persistent store, e.g. a database), whether or not in-memory state should be transferred depends on whether or not a "hot" cache is desired.

## 9.2. When State Transfer Occurs

If either in-memory or persistent state transfer is enabled, a full or partial state transfer will be done at various times, depending on how the cache is used. "Full" state transfer refers to the transfer of the state related to the entire tree -- i.e. the root node and all nodes below it. A "partial" state transfer is one where just a portion of the tree is transferred -- i.e. a node at a given Fqn and all nodes below it.

If either in-memory or persistent state transfer is enabled, state transfer will occur at the following times:

1. Initial state transfer. This occurs when the cache is first started (as part of the processing of the `start()` method). This is a full state transfer. The state is retrieved from the cache instance that has been operational the longest. If there is any problem receiving or integrating the state, the cache will not start.

   Initial state transfer will occur unless:

   1. The cache's `InactiveOnStartup` property is `true`. This property is used in conjunction with region-based marshalling; see below for more on this.

   2. Buddy replication is used. See below for more on state transfer with buddy replication.

2. Partial state transfer following region activation. Only relevant when region-based marshalling is used. Here a special classloader is needed to unmarshal the state for a portion of the tree. State transfer cannot succeed until the application registers this classloader with the cache. Once the application registers its classloader, it calls `activateRegion(String fqn)`. As part of the region activation process, a partial state transfer of the relevant subtree's state is performed. The state is requested from the oldest cache instance in the cluster; if that instance responds with no state, state is requested from each instance one by one until one provides state or all instances have been queried.

   Typically when region-based marshalling is used, the cache's `InactiveOnStartup` property is set to `true`. This suppresses initial state transfer, which would fail due to the inability to deserialize the transferred state.

3. Buddy replication. When buddy replication is used, initial state transfer is disabled. Instead, when a cache instance joins the cluster, it becomes the buddy of one or more other instances, and one or more other instances become its buddy. Each time an instance determines it has a new buddy providing backup for it, it pushes it's current state to the new buddy. This "pushing" of state to the new buddy is slightly different from other forms of state transfer, which are based on a "pull" approach (i.e. recipient asks for and receives state). However, the process of preparing and integrating the state is the same.

   This "push" of state upon buddy group formation only occurs if the `InactiveOnStartup` property is set to `false`. If it is `true`, state transfer amongst the buddies only occurs when the application calls `activateRegion(String fqn)` on the various members of the group.

   Partial state transfer following an `activateRegion()` call is slightly different in the buddy replication case as well. Instead of requesting the partial state from one cache instance, and trying all instances until one responds, with buddy replication the instance that is activating a region will request partial state from each instance for which it is serving as a backup.

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