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Storage Manager initial import

How to use Storage Manager with an existing kdb+ database

Storage Manager (SM) guarantees atomicity during write-down; and at the same time ensures that a database is mountable by vanilla kdb+ process at any point in time. To achieve this, SM uses symbolic links to represent a standard kdb+ segmented database, while keeping the backing data in a proprietary structure. Data in object storage is excluded from this transformation, and kept in standard kdb+ format.

Thus, to work with an existing database, SM first needs to adjust the database to its own format.

Import scenarios

Three scenarios are supported: - partitioned database on disk - partitions only in object storage - partitions on disk and partitions in object storage (the same date partition can't exist in both)

Configuration

Configure the SM to check for an existing kdb+ database under the elements.sm key within an assembly file. Once the SM has been initialized for the first time, and the database has been imported, this configuration can be removed. 

elements:
  sm:
    description: Storage manager
    source: stream
    initialImport: true
    tiers:
      - name: stream
        mount: rdb
      - name: idb
        mount: idb
        schedule:
          freq: 00:15:00      
      - name: hdb
        mount: hdb
        store: file:///data/hdb
        schedule:
          snap: 00:00:00           
        retain: 
          time: 2 weeks
      - name: objstor
        mount: hdb
        store: s3://historical-data/db
| name | type | required | description | | ---- | ---- | -------- | ----------- | | initialImport | boolean | No | When the flag is enabled the SM will check for an existing kdb+ database under the data sub-directory of the directory pointed to by baseURI of the HDB-based mount. If a database isn't found at the location the SM will terminate. After the first SM startup, the flag is redundant and can be removed.

Simple partitioned database on disk

The database is in the standard format for a partitioned (non-segmented) database. Put the database under the data sub-directory of the directory pointed to by baseURI of the HDB-based mount, that is, the mount whose type=local, and partition=date. The database is converted in-place to SM format.

mounts:
  rdb:
    type: stream
    partition: none
    baseURI: none
  idb:
    type: local
    partition: ordinal
    baseURI: file:///data/idb
  hdb:
    type: local
    partition: date
    baseURI: file:///data/hdb
Example schema definition 
tables:
  trade:
    description: Trade data
    type: partitioned
    prtnCol: time
    sortColsOrd: sym
    sortColsDisk: sym
    columns:
      - name: time
        description: Time
        type: timestamp
      - name: sym
        description: Symbol name
        type: symbol
        attrMem: grouped
        attrDisk: parted
        attrOrd: parted
      - name: price
        description: Price
        type: float
      - name: size
        description: Size
        type: long

Database structure

tree /data/hdb/data
├── 2024.01.01
│   └── trade
│       ├── price
│       ├── size
│       ├── sym
│       └── time
├── 2024.01.02
│   └── trade
│       ├── price
│       ├── size
│       ├── sym
│       └── time
└── sym

It is possible to have partitions located in object storage: set the store property of the last HDB-based tier to point to it (e.g. s3://historical-data/db), and SM will add an entry for it in the generated par.txt.

Partitions only in object storage

This scenario resembles the Simple partitioned database scenario, except that the location pointed to by the first HDB-based tier contains only the sym file (if applicable): all the partitions exist in object storage. SM will add an entry for it in the generated par.txt.

Database structure

tree data/hdb/data
data/hdb/data
└── sym

aws s3 ls s3://historical-data/db
                           PRE 2024.01.01/
                           PRE 2024.01.02/

Prerequisites

The following conditions must be met for all the above scenarios:

  • tables match the schema specified in the assembly configuration
  • partition values are date
  • no overlap between partition values (across tiers)
  • a backup copy of the data exists

Backup policy

Note that the backup is not enforced, since it is likely originating in a different volume before being copied to the SM volume. It is up to the user to ensure that this data is backed up somewhere prior to starting SM.

Future support

In the future, SM will support importing a fully segmented database, whose segments map one-to-one with tiers specified in the assembly configuration.

Database validation

The SM will validate the entire database against the schema configuration within the assembly to ensure that it conforms and is operational. If the SM validation finds any issues with the database it will provide details in the logs on what validation failed, and what needs to be addressed, before terminating. In this scenario the user can take SM offline and resolve the validation failures locally before attempting to re-initialize SM again.

Error recovery

SM has a recovery mechanism: if it gets interrupted during a long conversion, on restart it continues where it left off. If an error occurs during conversion, SM rolls back the database to its original state.