YEDB specifications
Version: 1
1. General
-
YEDB is the database format / engine, designed to store configuration files and other reliable data.
-
YEDB is designed for reliability: the data survive any power loss, unless the file system is broken.
-
YEDB is the data serialization / storage engine, used by Bohemia Automation and AlterTech in industrial and embedded software IoT/IIoT products.
-
YEDB has the free data format which can be used under Apache 2.0 software license.
-
YEDB uses a very simple data format: keys are stored as serialized regular files. The format helps the database survive disasters and is highly repairable.
-
YEDB libraries and client / server implementations are free, open-source and provided under Apache 2.0 software license.
1.1 Implementations
-
yedb-py - pure Python library and client/server. Extra features: byte objects, database serialization format conversion. "pickle" serialization format, which can store Python objects directly (embedded only).
-
yedb-rs - Rust version. Fast, reliable. Byte objects are not supported.
1.2 References
The following third-party technologies are mentioned in this document:
-
JSON: https://www.json.org
-
MessagePack: https://msgpack.org
-
CBOR: https://cbor.io
-
YAML: https://yaml.org
-
JSON RPC: https://www.jsonrpc.org
-
JSON Schema: https://json-schema.org
2. Database format
2.1 Common
-
all meta date/time values are represented as UNIX timestamps in nanoseconds (unsigned 64-bit integer).
-
all integer numbers are stored with little-endian order.
2.2 Meta information file
The file MUST be called ".yedb" and stored in the database root directory. The file MUST be serialized as JSON and contain the following fields:
| Name | Type | Description |
|---|---|---|
| fmt | String | data serialization format code |
| created | u64 | database creation timestamp |
| version | u16 | engine version |
| checksums | bool | data storage checksums enabled |
2.3 Key files
2.3.1 General
Key values MUST be stored in regular files in "keys" subdirectory. The format MUST be kept to allow system administrators repair a database without any external tools. The full path tree, where a key file is stored, MUST represent the key full name.
Example: a key, named "my/cool/key" should be:
-
stored in "keys/my/cool" directory
-
named "key" with the extension, representing the data serialization format
-
keys, which have the path starting with dot (".") are considered as hidden
When deleted, the engine SHOULD automatically remove unnecessary directories in the path tree.
The root key SHOULD NOT have any value. Other "parent" keys MAY have values set.
2.3.2 File format
The database can have files of the same format only. Checksums are enabled or disabled globally, for all keys in the database.
2.3.2.1 Checksums disabled
If checksums are disabled, key files contain serialized data as-is. This is more easy for manually repairing the database, but less reliable for data integrity.
2.3.2.2 Checksums enabled
2.3.2.2.1 Binary serialization formats
| Byte range | Size | Value |
|---|---|---|
| 0-31 | 32 | SHA256-checksum of serialized data |
| 32-39 | 8 | Set timestamp |
| 40- | Serialized data |
2.3.2.2.2 Text serialization formats
| String | Value |
|---|---|
| 1 | SHA256-checksum of serialized data (HEX) |
| 2 | Set timestamp (hex) |
| 3-N | Serialized data |
-
the strings MUST have no white spaces
-
the file MUST have UNIX (new line = LF) encoding
-
the serialized data SHOULD have LF ending to let key files be edited with regular text editors.
2.4 Data serialization
2.4.1 Data formats
The current YEDB specifications document defines the following serialization formats:
| Name | Code | Mandatory | File suffix | With c-sums | Type |
|---|---|---|---|---|---|
| json | 1 | Y(default) | .json | .jsonc | text |
| msgpack | 2 | Y | .mp | .mpc | binary |
| cbor | 3 | N | .cb | .cbc | binary |
| yaml | 4 | N | .yml | .ymlc | text |
2.4.2 Data types
| Name | Mandatory |
|---|---|
| null | Y |
| boolean | Y |
| number | Y |
| string | Y |
| array | Y |
| object | Y |
| bytes | N |
The database MAY implement additional data types.
2.4.3 Data type schemas
The database MAY implement strict type / structure checking for keys.
If implemented, the implementation MUST satisfy the following requirements:
-
Key schemas are defined as .schema/path/to/key or .schema/path keys.
-
A schema MUST be applied to individual keys or to all their sub keys, unless the lower-level schema is defined. E.g. the schema named .schema/group1 is applied to all group1 keys, unless some key group1/key1 has the individual schema at .schema/group1/key1.
-
The key schemas MUST implement JSON Schema.
-
The key schemas MAY extend JSON Schema and implement additional data types.
3. Engine
3.1 Basics
-
The database engine MUST support locking. Usually the locking can be performed with an exclusively-locked file, respected by other engine instances.
-
The default lock file SHOULD be called "db.lock" and created in the database root directory. The lock file SHOULD contain the owner process ID.
-
The engine MUST allow variable location of the lock file, allowing using of databases on read-only file systems.
-
The embedded engine MUST be thread-safe out-of-the-box or provide documentation how to implement thread-safe usage.
-
The embedded engine MAY offer asynchronous read/writes if keys are stored on different physical drives.
-
Client/server architecture is optional.
-
The engine MAY implement data caching for read operations ONLY.
-
Engine methods MAY accept keys with leading slashes ("/path/to/key"), but MUST represent all keys without them.
3.1.1 Writing and flushing data
If data flushing is enabled, the key and database data MUST be written into temporary files. After, these files MUST be flushed with the corresponding system call (e.g. fd.flush(); libc::fsync(fd)).
After the successful flushing:
-
the key MUST be renamed to its actual name
-
the key directory MUST be flushed as well
-
if "write_modified_only" parameter is set, the engine MUST make sure the key value is changed before writing any data to the disk.
3.2 Public database object variables
The database object MUST have the following variables, defined either as public or provide setters for them:
| Name | Type | Default | Description |
|---|---|---|---|
| auto_repair | bool | true | Auto-repair the database when opened |
| auto_flush | bool | true | Flush key data to disk immediately |
| lock_ ex | bool | true | Lock the database exclusively on open |
| write_modified_only | bool | true | Write to disk modified key values only |
3.3 Mandatory methods
| Name | Args | Brief description |
|---|---|---|
| purge | Remove all except key files and meta, delete broken keys | |
| safe_purge | The same as purge but do not delete broken keys | |
| repair | Repair broken keys, deletes unrepairable | |
| check | Check keys | |
| info | Get database info | |
| server_set | name: String, value: Value | Modify server options |
| key_exists | key: String | Return boolean True if the key exists; False if does not |
| key_get | key: String | Get key value |
| key_explain | key: String | Get key value and extended info |
| key_set | key: String, value | Set key value |
| key_decrement | key: String | Decrement values of numeric (integer) keys |
| key_delete | key: String | Delete the key |
| key_delete_recursive | key: String | Delete the key and all its subkeys |
| key_increment | key: String | Increment value of numeric (integer) keys |
| key_list | key: String | List the key and all its subkeys Vec |
| key_list_all | key: String | List key and all its subkeys, including hidden |
| key_get_recursive | key: String | Get the key value and all subkeys Vec<String, Value> |
| key_copy | key: String, dst_key: String | Copy the key value |
| key_rename | key: String, dst_key: String | Rename the key / key tree |
| key_dump | key: String | Get value of the key and all subkeys, ignores broken |
| key_load | data: Vec<String, Value> | Load dumped keys back |
3.3.1 Purge
The method MUST return a Generator<String> or an array/list of deleted broken keys.
3.3.2 Repair
The method MUST return a Generator<(String, bool)> or an array/list of deleted broken keys, where the bool value is true if the key is repaired and false if the key has been deleted.
3.3.3 Check
The method MUST return a Generator<String> or an array/list of broken keys.
3.3.4 Info
The method MUST return the following data object:
| Name | Type | Description | | ------------------- | ---------------- | ----------------------------------------------- --- | | auto_flush | bool | Flush key data to disk immediately | | checksums | bool | Checksums enabled | | created | u64 | Database creation timestamp | | fmt | String | Current data serialization format | | path | bool | Database path (server local) | | repair_recommended | bool | Database "repair recommended" flag (not auto-repaired) | | server | (String, String) | Server engine ID / Version (custom values) | | version | u16 | Engine version |
The object MAY contain additional fields.
3.3.5 Server set
The following server parameters are allowed to be modified on-the-flow:
| Name |
|---|
| auto_flush |
| repair_recommended |
3.3.6 Key Explain
The method MUST return the following data object:
| Name | Type | Description |
|---|---|---|
| file | String | Key file |
| schema | String | JSON schema key if schema is defined |
| len | u64 | length for strings, objects and arrays, null for others |
| mtime | u64 | Key file modification timestamp |
| stime | u64 | Key modification timestamp; null if checksums are disabled |
| sha256 | SHA256-checksum, MUST be serialized to String | |
| type | Value type (see 2.4.2), MUST be serialized to String | |
| value | Key value |
The object MAY contain additional fields.
If key explain is requested for a schema key, its "schema" field MUST be started with "!" symbol to inform clients that the key does not physically exist in the database.
If the database engine has data type schemas (see 2.4.3) implemented, the schema field for .schema keys MUST contain the value "!JSON Schema VERSION", e.g. "!JSON Schema Draft-7".
If the schema implements custom data types, this MUST be clearly and properly explained. E.g. if a key schema defines that keys must contain valid Python code, the value MUST contain either "Python" or the link to python.org.
3.3.7 Key increment and decrement methods
-
The methods SHOULD support working with at least 64-bit integer values.
-
The methods SHOULD return the new key value after successful increment/decrement.
4. Engine API
4.1 Basics
The engine MUST implement JSON RPC 2.0 API with the following conditions:
- batch requests: optional
- requests without a response required (with no "id" field): optional
- all mandatory engine methods MUST be provided
The engine MAY implement other APIs.
4.2 Test
The engine API MUST implement "test" method, which MUST return the following structure:
| Name | Type | Description |
|---|---|---|
| name | String | MUST have the value = "yedb" |
| version | u16 | Engine version |
4.3 Server types
| Type | Serialization formats | Notes |
|---|---|---|
| UNIX socket | msgpack | The name SHOULD have the suffix ".sock" or ".socket" |
| TCP socket | msgpack | The default port SHOULD be 8870 |
| HTTP | msgpack, json | The default port SHOULD be 8878 |
4.4 Binary packets format
For binary data exchange (UNIX/TCP sockets), the following format MUST be kept for both client and server:
| Byte range | Size | Value |
|---|---|---|
| 0 | 1 | Engine version |
| 1 | 1 | Data format code (2 for msgpack) |
| 2-5 | 4 | JSON RPC frame length (little-endian) |
| 6- | JSON RPC request / response frame |
4.5 HTTP
The HTTP transport MUST meet the following conditions:
- The API MUST respond to HTTP/POST requests at HTTP ROOT URI ("/")
- The API MUST accept JSON-encoded payloads by default
- The API MUST accept MessagePack-encoded payloads for requests with content type set to either "application/msgpack" or "application/x-msgpack".
4.6 JSON RPC Error codes
-
Error replies SHOULD include correct and clear error messages.
-
Schema validation errors MUST return detailed error messages.
Error codes, returned by server, MUST match the following:
4.6.1 Protocol errors
| Code | Description |
|---|---|
| -32600 | Invalid request |
| -32601 | Method not found |
| -32602 | Invalid method parameters |
4.6.2 Engine errors
| Code | Description |
|---|---|
| -32001 | Key not found |
| -32002 | Data error, checksum error |
| -32003 | Schema validation error |
| -32004 | OS I/O errors: device errors, permissions etc. |
| -32000 | All other server errors |
5. Dump files
If the engine or a client creates / loads dump files, these files MUST have data serialized with MessagePack and have the following format:
5.1 File header
| Byte range | Size | Value |
|---|---|---|
| 0 | 1 | Engine version |
| 1 | 1 | Data format code (2 for msgpack) |
5.2 Key data
Stored, starting from byte 2, for each key:
| Byte range | Size | Value |
|---|---|---|
| 0 | 4 | Data length |
| 4- | Data value (msgpack) |
