关于FlatBuffers Builder读写机制、SAX实现与文件压缩的技术咨询

Great questions—let’s break this down clearly, since FlatBuffers operates differently from XML/JSON tools you might be used to:

Core Behavior: All Builder Operations Are In-Memory (No Direct File I/O, No "DOM") #

First off, let’s correct a couple of terms: FlatBuffers doesn’t use a "DOM" structure (that’s specific to XML/JSON parsing) and it never performs direct, repeated file I/O when you’re using the schema-generated APIs to populate fields.

When you create a Builder instance and call methods like CreateString or populate struct fields, every single operation happens entirely in RAM. The builder constructs a compact binary buffer in memory as you add data—zero writes to disk happen until you explicitly serialize the final buffer to a file (or send it over a network, etc.).

On the reading side, you typically load the entire FlatBuffers binary into memory (or map it into memory via OS-level memory mapping) and access data directly from that buffer without parsing it into an intermediate object tree. This is one of FlatBuffers’ key performance features!

Handling Extremely Large Files: SAX-like Streaming Options #

If your dataset is too big to fit into RAM, FlatBuffers does support a streaming (SAX-like) approach for reading. Here’s how to use it:

• Ditch the high-level schema-generated APIs for reading, and use the low-level FlatBuffers API to manually parse elements as you read chunks of the file stream.
• A more practical approach for large datasets is frame-based streaming: split your data into smaller, self-contained FlatBuffers frames. Process each frame one at a time, loading only a single frame into memory at any point. This works well for log data, time-series datasets, etc.

FlatBuffers doesn’t have built-in streaming writers, but you can implement this by writing each completed frame to disk as you build it (instead of building everything in memory first).

Adding External Compression/Decompression #

FlatBuffers itself doesn’t handle compression—you’ll need to wrap your file I/O with a compression library of your choice (zlib, lz4, snappy, and zstd are all popular options). Here’s the standard workflow:

Writing with Compression #

1. Build your FlatBuffers buffer in memory using the Builder.
2. Extract the final buffer (via Builder.GetBufferPointer() and Builder.GetSize()).
3. Compress the buffer using your library of choice.
4. Write the compressed data to your output file.

Reading with Compression #

1. Read the compressed data from the file into a memory buffer (or read chunks incrementally for streaming).
2. Decompress the buffer (or chunk) to get the raw FlatBuffers binary.
3. Access the data using FlatBuffers’ reader API.

For streaming large compressed files, combine chunked reading, incremental decompression, and frame-based processing to avoid loading everything into RAM.

Example: When File I/O Actually Happens with FlatBuffers #

Here’s a C++ example that shows the exact points where file I/O occurs, plus integration with zlib for compression:

Writing to a Compressed File #

#include <flatbuffers/flatbuffers.h>
#include "your_schema_generated.h" // Generated from your .fbs schema
#include <zlib.h>
#include <fstream>
#include <vector>

// Helper to compress data with zlib
bool Compress(const uint8_t* input, size_t input_size, std::vector<uint8_t>& output) {
    z_stream zs = {0};
    if (deflateInit(&zs, Z_BEST_COMPRESSION) != Z_OK) return false;

    zs.next_in = const_cast<uint8_t*>(input);
    zs.avail_in = input_size;

    int ret;
    do {
        output.resize(output.size() + 1024);
        zs.next_out = output.data() + zs.total_out;
        zs.avail_out = output.size() - zs.total_out;
        ret = deflate(&zs, Z_FINISH);
    } while (ret == Z_OK);

    deflateEnd(&zs);
    if (ret != Z_STREAM_END) return false;

    output.resize(zs.total_out);
    return true;
}

int main() {
    // Step 1: ALL FlatBuffers work happens IN-MEMORY
    flatbuffers::FlatBufferBuilder builder;

    // Populate data using schema-generated methods
    auto item_name = builder.CreateString("Gigantic Dataset Item");
    auto data_item = CreateDataItem(builder, item_name, 98765);
    builder.Finish(data_item);

    // Step 2: Get the completed in-memory buffer
    const uint8_t* fb_buffer = builder.GetBufferPointer();
    size_t fb_size = builder.GetSize();

    // Step 3: Compress the buffer (still in memory)
    std::vector<uint8_t> compressed_buf;
    if (!Compress(fb_buffer, fb_size, compressed_buf)) {
        return 1;
    }

    // Step 4: ACTUAL FILE WRITE occurs here
    std::ofstream out_file("compressed_large_data.fb", std::ios::binary);
    out_file.write(reinterpret_cast<const char*>(compressed_buf.data()), compressed_buf.size());
    out_file.close();

    return 0;
}

Reading from a Compressed File #

#include <flatbuffers/flatbuffers.h>
#include "your_schema_generated.h"
#include <zlib.h>
#include <fstream>
#include <vector>

// Helper to decompress data with zlib
bool Decompress(const uint8_t* input, size_t input_size, std::vector<uint8_t>& output) {
    z_stream zs = {0};
    if (inflateInit(&zs) != Z_OK) return false;

    zs.next_in = const_cast<uint8_t*>(input);
    zs.avail_in = input_size;

    int ret;
    do {
        output.resize(output.size() + 1024);
        zs.next_out = output.data() + zs.total_out;
        zs.avail_out = output.size() - zs.total_out;
        ret = inflate(&zs, Z_NO_FLUSH);
    } while (ret == Z_OK);

    inflateEnd(&zs);
    if (ret != Z_STREAM_END) return false;

    output.resize(zs.total_out);
    return true;
}

int main() {
    // Step 1: ACTUAL FILE READ occurs here
    std::ifstream in_file("compressed_large_data.fb", std::ios::binary);
    std::vector<uint8_t> compressed_buf((std::istreambuf_iterator<char>(in_file)), {});
    in_file.close();

    // Step 2: Decompress into memory
    std::vector<uint8_t> fb_buffer;
    if (!Decompress(compressed_buf.data(), compressed_buf.size(), fb_buffer)) {
        return 1;
    }

    // Step 3: Access data IN-MEMORY via FlatBuffers reader
    auto data_item = GetDataItem(fb_buffer.data());
    printf("Item Name: %s, Value: %d\n", data_item->name()->c_str(), data_item->value());

    return 0;
}

Notice that all the FlatBuffers builder and reader logic stays in memory—file I/O only happens when we explicitly read or write the compressed buffer to disk.

内容的提问来源于stack exchange，提问作者Shivendra Agarwal