Hyperledger Fabric出块配置详解

Hyperledger Fabric的出块主要是Orderer节点负责，出块配置位于创世区块中，支持定时出块、达到一定交易数出块两种条件。出块配置位于configtx.yaml中，修改出块配置后需要重新生成创世区块。

相关参数

若需要修改fabric的出块机制，则需要调整以下配置参数：

• BatchTimeout：出块超时时间，最长出块间隔（但缓存中必须含有数据才会出块，否则无法出块，即fabric不会强行产生空块）
• MaxMessageCount：区块最大交易数量，当交易数量达到此参数后，会立即出块。
• PreferredMaxBytes：区块首选字节数，正常情况下一个区块中的交易数据大小会小于此参数。
• AbsoluteMaxBytes：区块最大字节数：所有情况下区块的最大允许字节数，超过此参数的交易将无法打包，直接退回。

出块机制和条件

基于上述指标，orderer会在两种条件下打包区块：

1.定时触发：维护以 BatchTimeout 为间隔的闹钟，定时检测缓存中是否还有未出块的交易，如果有则打包出块；

2.新交易触发：当满足条件的新交易与缓存中的交易大小之和与 PreferredMaxBytes 进行比较，超过此限制则将缓存中的交易进行打包，新交易进入缓存。或者交易数目超过MaxMessageCount，也会进行打包。

默认参数配置

fabric的reslease-2.0分支中默认出块配置如下：

    # Batch Timeout: The amount of time to wait before creating a batch.
    BatchTimeout: 2s

    # Batch Size: Controls the number of messages batched into a block.
    # The orderer views messages opaquely, but typically, messages may
    # be considered to be Fabric transactions.  The 'batch' is the group
    # of messages in the 'data' field of the block.  Blocks will be a few kb
    # larger than the batch size, when signatures, hashes, and other metadata
    # is applied.
    BatchSize:

        # Max Message Count: The maximum number of messages to permit in a
        # batch.  No block will contain more than this number of messages.
        MaxMessageCount: 500

        # Absolute Max Bytes: The absolute maximum number of bytes allowed for
        # the serialized messages in a batch. The maximum block size is this value
        # plus the size of the associated metadata (usually a few KB depending
        # upon the size of the signing identities). Any transaction larger than
        # this value will be rejected by ordering. If the "kafka" OrdererType is
        # selected, set 'message.max.bytes' and 'replica.fetch.max.bytes' on
        # the Kafka brokers to a value that is larger than this one.
        AbsoluteMaxBytes: 10 MB

        # Preferred Max Bytes: The preferred maximum number of bytes allowed
        # for the serialized messages in a batch. Roughly, this field may be considered
        # the best effort maximum size of a batch. A batch will fill with messages
        # until this size is reached (or the max message count, or batch timeout is
        # exceeded).  If adding a new message to the batch would cause the batch to
        # exceed the preferred max bytes, then the current batch is closed and written
        # to a block, and a new batch containing the new message is created.  If a
        # message larger than the preferred max bytes is received, then its batch
        # will contain only that message.  Because messages may be larger than
        # preferred max bytes (up to AbsoluteMaxBytes), some batches may exceed
        # the preferred max bytes, but will always contain exactly one transaction.
        PreferredMaxBytes: 2 MB