ESPJsonDB

A lightweight document database for ESP32 devices. ESPJsonDB borrows the ergonomics of MongoDB/Mongoose while embracing embedded constraints: collections live as JSON on LittleFS, memory use is capped through an optional cache, and every API leans on ArduinoJson types so you can stay inside a single document representation.

CI / Release / License

Features

• Simple, mongoose-like API for embedded projects (create/update/remove/find with predicates or JSON filters).
• Optional in-memory cache with dirty-document tracking and change detection to avoid needless flash I/O.
• Automatic LittleFS synchronisation on a background FreeRTOS task (ESPJsonDBConfig controls interval, stack, priority, and core affinity).
• MessagePack compression + StreamUtils for efficient read/write pipelines.
• Schema registry with required fields, defaults, type validation, and collection-level unique constraints.
• Event + error callbacks so firmware can observe sync cycles or take action when validation fails.
• Snapshot/restore helpers for backups plus diagnostics that report per-collection counts and config details.
• Generic file storage helpers under /<baseDir>/_files with chunked stream read/write for any file type (text, binary, etc.).

Examples

Quick start:

#include <ESPJsonDB.h>

ESPJsonDB db;

void setup() {
    Serial.begin(115200);

    ESPJsonDBConfig cfg;
    cfg.intervalMs = 3000;  // autosync every 3s
    cfg.autosync = true;
    cfg.usePSRAMBuffers = true; // optional: prefer PSRAM for internal byte buffers

    if (!db.init("/test_db", cfg).ok()) {
        Serial.println("DB init failed");
        return;
    }

    db.onEvent([](DBEventType evt){
        Serial.printf("Event: %s\n", dbEventTypeToString(evt));
    });
    db.onError([](const DbStatus &st){
        Serial.printf("Error: %s\n", st.message);
    });
}

Working with documents is intentionally JsonDocument-centric:

JsonDocument doc;
doc["email"] = "user@example.com";
doc["role"] = "admin";
auto createRes = db.create("users", doc.as<JsonObjectConst>());

if (createRes.status.ok()) {
    const std::string& id = createRes.value;
    auto found = db.findById("users", id);
    if (found.status.ok()) {
        Serial.printf("Role: %s\n", found.value["role"].as<const char*>());
    }
    db.updateById("users", id, [](DocView& view){
        view["role"].set("owner");
    });
    db.removeById("users", id);
}

See the sketches under examples/ for end-to-end flows:

• QuickStart – database initialisation and simple CRUD.
• Collections – create/drop collections at runtime.
• CacheDisabled – run without the RAM cache.
• BulkOperations – batch inserts, updates, and queries.
• SchemaValidation – enforce required fields and custom validators.
• UniqueFields – per-collection uniqueness guarantees.
• References – store one-to-many relations and populate them lazily.
• FileStreaming – store and stream txt / json / csv / bin / custom extension payloads.
• LargeFileStreaming – chunked upload + chunked verification for a large binary payload without full-buffer RAM copies.
• AsyncFileUpload – non-blocking, callback-driven chunk upload on a background task.
• AsyncLargeFileUpload – background chunk upload for a large binary payload with progress polling and streaming hash verification.

File storage example:

ESPJsonDBFileOptions fileOpts;
fileOpts.chunkSize = 256;
db.writeTextFile("notes/readme.txt", "hello from esp-jsondb");

db.writeFileFromPath("firmware/chunk.bin", "/fw/chunk.bin", fileOpts);

db.writeFileStream(
    "firmware/chunk_cb.bin",
    [](size_t requested, uint8_t* buffer, size_t& produced, bool& eof) -> DbStatus {
        // fill `buffer` with up to `requested` bytes, set produced/eof
        produced = 0;
        eof = true;
        return {DbStatusCode::Ok, ""};
    },
    fileOpts
);

Gotchas

• Each collection lives in RAM when the cache is enabled; disable the cache or add PSRAM when handling large documents.
• All payloads are JSON; converting to structs is optional but deserialisation still costs memory—size your JsonDocument objects carefully.
• Sync callbacks run on the background task; keep them short to avoid blocking periodic flushes.
• Unique constraints and validators run inside write operations. Long-running validators will increase latency for the calling task.
• writeFileStream() and readFileStream() hold the filesystem lock while processing the stream; use reasonable chunk sizes and avoid blocking stream sources/sinks.
• writeFileStreamAsync() runs producer callbacks on a background task; callbacks must be short and thread-safe.
• /_files is an internal reserved directory used for file storage and cannot be used as a collection name.
• getSnapshot() and restoreFromSnapshot() currently cover document collections only; file storage under /_files is not included.
• usePSRAMBuffers affects ESPJsonDB-owned byte buffers and decoded DocView JsonDocument pools on ArduinoJson v7. Public return containers like readFile() still use the existing API types.

API Reference

• DbStatus init(const char* baseDir = "/db", const ESPJsonDBConfig& cfg = {}) – mount LittleFS (cfg.initFileSystem) and create the autosync task (optional). Diagnostics are lightweight and served from in-memory counters.
• void onEvent(std::function<void(DBEventType)>) / void onError(std::function<void(const DbStatus&)>) – receive sync, CRUD, and validation events.
• Collection management: collection(name), dropCollection(name), dropAll(), getAllCollectionName().
• Document helpers:
  • Create: create, createMany (JSON array) plus direct Collection::create* variants.
  • Read: findById, findOne, findMany (predicate or JSON filter) returning DocView so you can read/write lazily.
  • Update/delete: updateOne, updateById, updateMany, removeById, removeMany (predicate or JSON filter).
• Schemas: registerSchema(name, Schema), unRegisterSchema(name); Schema exposes fields with type/default/unique flags plus optional custom validate callables.
• References: store { "collection": "authors", "_id": "..." } inside a document and call DocView::populate(fieldName) to expand the reference into an embedded object.
• Sync + diagnostics: syncNow(), getDiag() (JSON summary), getSnapshot() / restoreFromSnapshot() for backups.
  • getDiag() does not touch the filesystem; it reports cached counters overlaid with currently loaded collection sizes.
• File storage:
  • writeFileStream(path, in, bytesToWrite, opts) / readFileStream(path, out, chunkSize) for chunked stream transfer.
  • writeFileStream(path, pullCb, opts) for synchronous callback-driven chunk production.
  • writeFileFromPath(path, sourceFsPath, opts) to copy a source file path into DB-managed file storage.
  • writeFileStreamAsync(path, pullCb, opts, doneCb) for non-blocking producer-driven uploads.
  • cancelFileUpload(uploadId), getFileUploadState(uploadId) for async job control.
  • writeFile(path, data, size) / readFile(path) for direct byte buffers.
  • writeTextFile(path, text) / readTextFile(path) for UTF-8 or plain text payloads.
  • fileExists(path), fileSize(path), removeFile(path) for file lifecycle utilities.
  • File paths are relative to /<baseDir>/_files and path traversal segments are rejected.
  • ESPJsonDBFileOptions: overwrite and chunkSize controls for stream writes.
  • DbFileUploadPullCb: callback receives (requested, buffer, produced, eof) and fills bytes into buffer.

ESPJsonDBConfig knobs:

• intervalMs, stackSize, priority, coreId – background autosync cadence & FreeRTOS tuning.
• autosync, coldSync, cacheEnabled – enable/disable timers and caches.
• fs, initFileSystem, formatOnFail, partitionLabel, maxOpenFiles – file system integration; pass your own fs::FS if you mount LittleFS elsewhere.
• usePSRAMBuffers – prefer PSRAM for internal msgpack + file stream byte buffers and decoded DocView JsonDocument pools (ArduinoJson v7), with safe fallback to default heap. Task stacks are always created from internal RAM.

Stack sizes are expressed in bytes.

Restrictions

• Designed for ESP32 + LittleFS. Other platforms/FSes are untested.
• Large documents are only practical on boards with PSRAM when the cache is enabled.
• Requires ArduinoJson 6+, StreamUtils, and a FreeRTOS-capable environment (Arduino-ESP32 or ESP-IDF with C++17).

Tests

An integration harness (test/) runs CRUD, bulk, schema, reference, and diagnostic scenarios via the DbTester class. Build it as a PlatformIO test or ESP-IDF component (include test/dbTest.cpp in your project) and run it on hardware to validate changes. Contributions that expand automated coverage are welcome.

License

MIT — see LICENSE.md.

ESPToolKit

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