Append Only Table #

If a table does not have a primary key defined, it is an append-only table by default. Separated by the definition of bucket, we have two different append-only mode: “Append For Scalable Table” and “Append For Queue”.

You can only insert a complete record into the table. No delete or update is supported, and you cannot define primary keys. This type of table is suitable for use cases that do not require updates (such as log data synchronization).

Append For Scalable Table #

Definition #

By defining 'bucket' = '-1' in table properties, you can assign a special mode (we call it “unaware-bucket mode”) to this table (see Example). In this mode, all the things are different. We don’t have the concept of bucket anymore, and we don’t guarantee the order of streaming read. We regard this table as a batch off-line table ( although we can stream read and write still). All the records will go into one directory (for compatibility, we put them in bucket-0), and we do not maintain the order anymore. As we don’t have the concept of bucket, we will not shuffle the input records by bucket anymore, which will speed up the inserting.

Using this mode, you can replace your Hive table to lake table.

Compaction #

In unaware-bucket mode, we don’t do compaction in writer, instead, we use Compact Coordinator to scan the small files and submit compaction task to Compact Worker. By this, we can easily do compaction for one simple data directory in parallel. In streaming mode, if you run insert sql in flink, the topology will be like this:

It will do its best to compact small files, but when a single small file in one partition remains long time and no new file added to the partition, the Compact Coordinator will remove it from memory to reduce memory usage. After you restart the job, it will scan the small files and add it to memory again. The options to control the compact behavior is exactly the same as Append For Qeueue. If you set write-only to true, the Compact Coordinator and Compact Worker will be removed in the topology.

The auto compaction is only supported in Flink engine streaming mode. You can also start a compaction job in flink by flink action in paimon and disable all the other compaction by set write-only.

Sort Compact #

The data in a per-partition out of order will lead a slow select, compaction may slow down the inserting. It is a good choice for you to set write-only for inserting job, and after per-partition data done, trigger a partition Sort Compact action. See Sort Compact.

Streaming Source #

Unaware-bucket mode append-only table supported streaming read and write, but no longer guarantee order anymore. You cannot regard it as a queue, instead, as a lake with storage bins. Every commit will generate a new record bin, we can read the increase by reading the new record bin, but records in one bin are flowing to anywhere they want, and we fetch them in any possible order. While in the Append For Queue mode, records are not stored in bins, but in record pipe. We can see the difference below.

Streaming Multiple Partitions Write #

Since the number of write tasks that Paimon-sink needs to handle is: the number of partitions to which the data is written * the number of buckets per partition. Therefore, we need to try to control the number of write tasks per paimon-sink task as much as possible,so that it is distributed within a reasonable range. If each sink-task handles too many write tasks, not only will it cause problems with too many small files, but it may also lead to out-of-memory errors.

In addition, write failures introduce orphan files, which undoubtedly adds to the cost of maintaining paimon. We need to avoid this problem as much as possible.

For flink-jobs with auto-merge enabled, we recommend trying to follow the following formula to adjust the parallelism of paimon-sink(This doesn’t just apply to append-only-tables, it actually applies to most scenarios):

(N*B)/P < 100   (This value needs to be adjusted according to the actual situation)
N(the number of partitions to which the data is written)
B(bucket number)
P(parallelism of paimon-sink)
100 (This is an empirically derived threshold,For flink-jobs with auto-merge disabled, this value can be reduced.
However, please note that you are only transferring part of the work to the user-compaction-job, you still have to deal with the problem in essence,
the amount of work you have to deal with has not been reduced, and the user-compaction-job still needs to be adjusted according to the above formula.)

You can also set write-buffer-spillable to true, writer can spill the records to disk. This can reduce small files as much as possible.To use this option, you need to have a certain size of local disks for your flink cluster. This is especially important for those using flink on k8s.

For append-only-table,You can set write-buffer-for-append option for append-only table. Setting this parameter to true, writer will cache the records use Segment Pool to avoid OOM.

Example #

The following is an example of creating the Append-Only table and specifying the bucket key.

CREATE TABLE MyTable (
    product_id BIGINT,
    price DOUBLE,
    sales BIGINT
) WITH (
    'bucket' = '-1'
);

Append For Queue #

Definition #

In this mode, you can regard append-only table as a queue separated by bucket. Every record in the same bucket is ordered strictly, streaming read will transfer the record to down-stream exactly in the order of writing. To use this mode, you do not need to config special configurations, all the data will go into one bucket as a queue. You can also define the bucket and bucket-key to enable larger parallelism and disperse data (see Example).

Compaction #

By default, the sink node will automatically perform compaction to control the number of files. The following options control the strategy of compaction:

Streaming Source #

Streaming source behavior is only supported in Flink engine at present.

Streaming Read Order #

For streaming reads, records are produced in the following order:

• For any two records from two different partitions
  • If scan.plan-sort-partition is set to true, the record with a smaller partition value will be produced first.
  • Otherwise, the record with an earlier partition creation time will be produced first.
• For any two records from the same partition and the same bucket, the first written record will be produced first.
• For any two records from the same partition but two different buckets, different buckets are processed by different tasks, there is no order guarantee between them.

Watermark Definition #

You can define watermark for reading Paimon tables:

CREATE TABLE T (
    `user` BIGINT,
    product STRING,
    order_time TIMESTAMP(3),
    WATERMARK FOR order_time AS order_time - INTERVAL '5' SECOND
) WITH (...);

-- launch a bounded streaming job to read paimon_table
SELECT window_start, window_end, COUNT(`user`) FROM TABLE(
 TUMBLE(TABLE T, DESCRIPTOR(order_time), INTERVAL '10' MINUTES)) GROUP BY window_start, window_end;

You can also enable Flink Watermark alignment, which will make sure no sources/splits/shards/partitions increase their watermarks too far ahead of the rest:

Bounded Stream #

Streaming Source can also be bounded, you can specify ‘scan.bounded.watermark’ to define the end condition for bounded streaming mode, stream reading will end until a larger watermark snapshot is encountered.

Watermark in snapshot is generated by writer, for example, you can specify a kafka source and declare the definition of watermark. When using this kafka source to write to Paimon table, the snapshots of Paimon table will generate the corresponding watermark, so that you can use the feature of bounded watermark when streaming reads of this Paimon table.

CREATE TABLE kafka_table (
    `user` BIGINT,
    product STRING,
    order_time TIMESTAMP(3),
    WATERMARK FOR order_time AS order_time - INTERVAL '5' SECOND
) WITH ('connector' = 'kafka'...);

-- launch a streaming insert job
INSERT INTO paimon_table SELECT * FROM kakfa_table;

-- launch a bounded streaming job to read paimon_table
SELECT * FROM paimon_table /*+ OPTIONS('scan.bounded.watermark'='...') */;

Example #

The following is an example of creating the Append-Only table and specifying the bucket key.

CREATE TABLE MyTable (
    product_id BIGINT,
    price DOUBLE,
    sales BIGINT
) WITH (
    'bucket' = '8',
    'bucket-key' = 'product_id'
);