Package org.apache.storm.blobstore

Class KeySequenceNumber

  • public class KeySequenceNumber
extends Object
    Class hands over the key sequence number which implies the number of updates made to a blob. The information regarding the keys and the sequence number which represents the number of updates are stored within the zookeeper in the following format. /storm/blobstore/key_name/nimbushostport-sequencenumber Example: If there are two nimbodes with nimbus.seeds:leader,non-leader are set, then the state inside the zookeeper is eventually stored as: /storm/blobstore/key1/leader:8080-1 /storm/blobstore/key1/non-leader:8080-1 indicates that a new blob with the name key1 has been created on the leader nimbus and the non-leader nimbus syncs after a call back is triggered by attempting to download the blob and finally updates its state inside the zookeeper.

    A watch is placed on the /storm/blobstore/key1 and the znodes leader:8080-1 and non-leader:8080-1 are ephemeral which implies that these nodes exist only until the connection between the corresponding nimbus and the zookeeper persist. If in case the nimbus crashes the node disappears under /storm/blobstore/key1.

    The sequence number for the keys are handed over based on the following scenario: Lets assume there are three nimbodes up and running, one being the leader and the other being the non-leader.

    1. Create is straight forward. Check whether the znode -> /storm/blobstore/key1 has been created or not. It implies the blob has not been created yet. If not created, it creates it and updates the zookeeper states under /storm/blobstore/key1 and /storm/blobstoremaxkeysequencenumber/key1. The znodes it creates on these nodes are /storm/blobstore/key1/leader:8080-1, /storm/blobstore/key1/non-leader:8080-1 and /storm/blobstoremaxkeysequencenumber/key1/1. The latter holds the global sequence number across all nimbodes more like a static variable indicating the true value of number of updates for a blob. This node helps to maintain sanity in case leadership changes due to crashing.

    2. Delete does not require to hand over the sequence number.

    3. Finally, the update has few scenarios.

    The class implements a TreeSet. The basic idea is if all the nimbodes have the same sequence number for the blob, then the number of elements in the set is 1 which holds the latest value of sequence number. If the number of elements are greater than 1 then it implies that there is sequence mismatch and there is need for syncing the blobs across nimbodes.

    The logic for handing over sequence numbers based on the state are described as follows Here consider Nimbus-1 alias as N1 and Nimbus-2 alias as N2. Scenario 1: Example: Normal create/update scenario Operation Nimbus-1:state Nimbus-2:state Seq-Num-Nimbus-1 Seq-Num-Nimbus-2 Max-Seq-Num Create-Key1 alive - Leader alive 1 1 Sync alive - Leader alive 1 1 (callback -> download) 1 Update-Key1 alive - Leader alive 2 1 2 Sync alive - Leader alive 2 2 (callback -> download) 2

    Scenario 2: Example: Leader nimbus crash followed by leader election, update and ex-leader restored again Operation Nimbus-1:state Nimbus-2:state Seq-Num-Nimbus-1 Seq-Num-Nimbus-2 Max-Seq-Num Create alive - Leader alive 1 1 Sync alive - Leader alive 1 1 (callback -> download) 1 Update alive - Leader alive 2 1 2 Sync alive - Leader alive 2 2 (callback -> download) 2 Update alive - Leader alive 3 2 3 Crash crash - Leader alive 3 2 3 New - Leader crash alive - Leader 3 (Invalid) 2 3 Update crash alive - Leader 3 (Invalid) 4 (max-seq-num + 1) 4 N1-Restored alive alive - Leader 0 4 4 Sync alive alive - Leader 4 4 4

    Scenario 3: Example: Leader nimbus crash followed by leader election, update and ex-leader restored again Operation Nimbus-1:state Nimbus-2:state Seq-Num-Nimbus-1 Seq-Num-Nimbus-2 Max-Seq-Num Create alive - Leader alive 1 1 Sync alive - Leader alive 1 1 (callback -> download) 1 Update alive - Leader alive 2 1 2 Sync alive - Leader alive 2 2 (callback -> download) 2 Update alive - Leader alive 3 2 3 Crash crash - Leader alive 3 2 3 Elect Leader crash alive - Leader 3 (Invalid) 2 3 N1-Restored alive alive - Leader 3 2 3 Read/Update alive alive - Leader 3 4 (Downloads from N1) 4 Sync alive alive - Leader 4 (callback) 4 4 Here the download is triggered whenever an operation corresponding to the blob is triggered on the nimbus like a read or update operation. Here, in the read/update call it is hard to know which call is read or update. Hence, by incrementing the sequence number to max-seq-num + 1 we ensure that the synchronization happens appropriately and all nimbodes have the same blob.

    • Constructor Detail

      • KeySequenceNumber

        public KeySequenceNumber​(String key,
                         NimbusInfo nimbusInfo)

    • Method Detail