CounterContext (apache-cassandra API)

java.lang.Object
- org.apache.cassandra.db.context.CounterContext

```
public class CounterContext
extends java.lang.Object
```
An implementation of a partitioned counter context. A context is primarily a list of tuples (counter id, clock, count) -- called shards, with some shards flagged as global or local (with special resolution rules in merge()). The data structure has two parts: a) a header containing the lists of global and local shard indexes in the body b) a list of shards -- (counter id, logical clock, count) tuples -- (the so-called 'body' below) The exact layout is: | header | body | context : |--|------|----------| ^ ^ | list of indices in the body list (2*#elt bytes) #elt in rest of header (2 bytes) Non-negative indices refer to local shards. Global shard indices are encoded as [idx + Short.MIN_VALUE], and are thus always negative. The body layout being: body: |----|----|----|----|----|----|.... ^ ^ ^ ^ ^ ^ | | count_1 | | count_2 | clock_1 | clock_2 counterid_1 counterid_2 The rules when merging two shard with the same counter id are: - global + global = keep the shard with the highest logical clock - global + local = keep the global one - global + remote = keep the global one - local + local = sum counts (and logical clocks) - local + remote = keep the local one - remote + remote = keep the shard with the highest logical clock For a detailed description of the meaning of a local and why the merging rules work this way, see CASSANDRA-1938 - specifically the 1938_discussion attachment (doesn't cover global shards, see CASSANDRA-4775 for that).

Nested Class Summary

Nested Classes
Modifier and Type	Class and Description
`static class`	`CounterContext.ContextState` Helper class to work on contexts (works by iterating over them).
`static class`	`CounterContext.Relationship`

Constructor Summary

Constructors
Constructor and Description

CounterContext()

Constructors
Constructor and Description
`CounterContext()`

Method Summary

Methods
Modifier and Type	Method and Description
`java.nio.ByteBuffer`	`clearAllLocal(java.nio.ByteBuffer context)` Remove all the local of a context (but keep global).
`java.nio.ByteBuffer`	`createGlobal(CounterId id, long clock, long count)` Creates a counter context with a single global, 2.1+ shard (a result of increment).
`java.nio.ByteBuffer`	`createLocal(long count)` Creates a counter context with a single local shard.
`java.nio.ByteBuffer`	`createRemote(CounterId id, long clock, long count)` Creates a counter context with a single remote shard.
`CounterContext.Relationship`	`diff(java.nio.ByteBuffer left, java.nio.ByteBuffer right)` Determine the count relationship between two contexts.
`int`	`findPositionOf(java.nio.ByteBuffer context, CounterId id)` Finds the position of a shard with the given id within the context (via binary search).
`ClockAndCount`	`getClockAndCountOf(java.nio.ByteBuffer context, CounterId id)` Returns the clock and the count associated with the given counter id, or (0, 0) if no such shard is present.
`ClockAndCount`	`getLocalClockAndCount(java.nio.ByteBuffer context)` Returns the clock and the count associated with the local counter id, or (0, 0) if no such shard is present.
`boolean`	`hasLegacyShards(java.nio.ByteBuffer context)` Detects whether or not the context has any legacy (local or remote) shards in it.
`static CounterContext`	`instance()`
`java.nio.ByteBuffer`	`markLocalToBeCleared(java.nio.ByteBuffer context)` Mark context to delete local references afterward.
`java.nio.ByteBuffer`	`merge(java.nio.ByteBuffer left, java.nio.ByteBuffer right)` Return a context w/ an aggregated count for each counter id.
`boolean`	`shouldClearLocal(java.nio.ByteBuffer context)`
`java.lang.String`	`toString(java.nio.ByteBuffer context)` Human-readable String from context.
`long`	`total(java.nio.ByteBuffer context)` Returns the aggregated count across all counter ids.
`void`	`updateDigest(java.security.MessageDigest message, java.nio.ByteBuffer context)` Update a MessageDigest with the content of a context.
`void`	`validateContext(java.nio.ByteBuffer context)`

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Constructor Detail
  - CounterContext
```
public CounterContext()
```
- Method Detail
  - instance
```
public static CounterContext instance()
```
  - createGlobal
```
public java.nio.ByteBuffer createGlobal(CounterId id,
                               long clock,
                               long count)
```
    Creates a counter context with a single global, 2.1+ shard (a result of increment).
  - createLocal
```
public java.nio.ByteBuffer createLocal(long count)
```
    Creates a counter context with a single local shard. For use by tests of compatibility with pre-2.1 counters only.
  - createRemote
```
public java.nio.ByteBuffer createRemote(CounterId id,
                               long clock,
                               long count)
```
    Creates a counter context with a single remote shard. For use by tests of compatibility with pre-2.1 counters only.
  - diff
```
public CounterContext.Relationship diff(java.nio.ByteBuffer left,
                               java.nio.ByteBuffer right)
```
    Determine the count relationship between two contexts. EQUAL: Equal set of nodes and every count is equal. GREATER_THAN: Superset of nodes and every count is equal or greater than its corollary. LESS_THAN: Subset of nodes and every count is equal or less than its corollary. DISJOINT: Node sets are not equal and/or counts are not all greater or less than. Strategy: compare node logical clocks (like a version vector).
    
    Parameters:
    left - counter context.
    right - counter context.
    
    Returns:
    the Relationship between the contexts.
  - merge
```
public java.nio.ByteBuffer merge(java.nio.ByteBuffer left,
                        java.nio.ByteBuffer right)
```
    Return a context w/ an aggregated count for each counter id.
    
    Parameters:
    left - counter context.
    right - counter context.
  - toString
```
public java.lang.String toString(java.nio.ByteBuffer context)
```
    Human-readable String from context.
    
    Parameters:
    context - counter context.
    
    Returns:
    a human-readable String of the context.
  - total
```
public long total(java.nio.ByteBuffer context)
```
    Returns the aggregated count across all counter ids.
    
    Parameters:
    context - a counter context
    
    Returns:
    the aggregated count represented by context
  - shouldClearLocal
```
public boolean shouldClearLocal(java.nio.ByteBuffer context)
```
  - hasLegacyShards
```
public boolean hasLegacyShards(java.nio.ByteBuffer context)
```
    Detects whether or not the context has any legacy (local or remote) shards in it.
  - markLocalToBeCleared
```
public java.nio.ByteBuffer markLocalToBeCleared(java.nio.ByteBuffer context)
```
    Mark context to delete local references afterward. Marking is done by multiply #elt by -1 to preserve header length and #elt count in order to clear all local refs later.
    
    Parameters:
    context - a counter context
    
    Returns:
    context that marked to delete local refs
  - clearAllLocal
```
public java.nio.ByteBuffer clearAllLocal(java.nio.ByteBuffer context)
```
    Remove all the local of a context (but keep global).
    
    Parameters:
    context - a counter context
    
    Returns:
    a version of context where no shards are local.
  - validateContext
```
public void validateContext(java.nio.ByteBuffer context)
                     throws MarshalException
```
    Throws:
    
    MarshalException
  - updateDigest
```
public void updateDigest(java.security.MessageDigest message,
                java.nio.ByteBuffer context)
```
    Update a MessageDigest with the content of a context. Note that this skips the header entirely since the header information has local meaning only, while digests are meant for comparison across nodes. This means in particular that we always have: updateDigest(ctx) == updateDigest(clearAllLocal(ctx))
  - getLocalClockAndCount
```
public ClockAndCount getLocalClockAndCount(java.nio.ByteBuffer context)
```
    Returns the clock and the count associated with the local counter id, or (0, 0) if no such shard is present.
  - getClockAndCountOf
```
public ClockAndCount getClockAndCountOf(java.nio.ByteBuffer context,
                               CounterId id)
```
    Returns the clock and the count associated with the given counter id, or (0, 0) if no such shard is present.
  - findPositionOf
```
public int findPositionOf(java.nio.ByteBuffer context,
                 CounterId id)
```
    Finds the position of a shard with the given id within the context (via binary search).

Class CounterContext

Nested Class Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Detail

CounterContext

Method Detail

instance

createGlobal

createLocal

createRemote

diff

merge

toString

total

shouldClearLocal

hasLegacyShards

markLocalToBeCleared

clearAllLocal

validateContext

updateDigest

getLocalClockAndCount

getClockAndCountOf

findPositionOf