[BlobStorage] Adding explainer for blob storage system.

storage/browser/blob/README.md

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+# Chrome's Blob Storage System Design
+
+Elaboration of the blob storage system in Chrome.
+
+# What are blobs?
+
+Please see the [FileAPI Spec](https://www.w3.org/TR/FileAPI/) for the full
+specification for Blobs, or [Mozilla's Blob documentation](
+https://developer.mozilla.org/en-US/docs/Web/API/Blob) for a description of how
+Blobs are used in the Web Platform in general. For the purposes of this
+document, the important aspects of blobs are:
+
+1. Blobs are immutable.
+2. Blob can be made using one or more of: bytes, files, or other blobs.
+3. Blobs can be ['sliced'](
+https://developer.mozilla.org/en-US/docs/Web/API/Blob/slice), which creates a
+blob that is a subsection of another blob.
+4. Reading blobs is asynchronous.
+5. Reading blob metadata (like size) is synchronous.
+6. Blobs can be passed to other browsing contexts, such as Javascript workers
+or other tabs.
+
+In Chrome, after blob creation the actual blob 'data' gets transported to and
+lives in the browser process. The renderer just holds a reference -
+specifically a string UUID - to the blob, which it can use to read the blob or
+pass it to other processes.
+
+# Summary & Terminology
+
+Blobs are created in a renderer process, where their data is temporarily held
+for the browser (while Javascript execution can continue). When the browser has
+enough memory quota for the blob, it requests the data from the renderer. All
+blob data is transported from the renderer to the browser. Once complete, any
+pending reads for the blob are allowed to complete. Blobs can be huge (GBs), so
+quota is necessary.
+
+If the in-memory space for blobs is getting full, or a new blob is too large to
+be in-memory, then the blob system uses the disk. This can either be paging old
+blobs to disk, or saving the new too-large blob straight to disk.
+
+Blob reading goes through the network layer, where the renderer dispatches a
+network request for the blob and the browser responds with the
+`BlobURLRequestJob`.
+
+General Chrome terminology:
+
+* **Renderer, Browser, and IPCs**: See the [Multi-Process Architecture](
+https://www.chromium.org/developers/design-documents/multi-process-architecture)
+document to learn about these concepts.
+* **Shared Memory**: Memory that both the browser and renderer process can read
+& write. Created only between 2 processes.
+
+Blob system terminology:
+
+* **Blob**: This is a blob object, which can consist of bytes or files, as
+described above.
+* **BlobItem** or **[DataElement](
+https://cs.chromium.org/chromium/src/storage/common/data_element.h)**:
+This is a primitive element that can basically be a File, Bytes, or another
+Blob. It also stores an offset and size, so this can be a part of a file. (This
+can also represent a "future" file and "future" bytes, which is used to signify
+a bytes or file item that has not been transported yet).
+* **dependent blobs**: These are blobs that our blob is dependent on to be
+constructed. As in, a blob is constructed with a dependency on another blob
+(maybe it is a slice or just a blob in our constructor), and before the new
+blob can be constructed it might need to wait for the "dependent" blobs to
+complete. (This can sound backwards, but it's how it's referenced in the code.
+So think "I am dependent on these other blobs")
+* **transportation strategy**: a method for sending the data in a BlobItem from
+a renderer to the browser. The system currently implements three strategies:
+IPC, Shared Memory, and Files.
+* **blob description**: the inital data sychronously sent to the browser that
+describes the items (content and sizes) of the new blob. This can
+optimistically include the blob data if the size is less than the maximimum IPC
+size.
+
+# Blob Storage Limits
+
+We calculate the storage limits [here](
+https://cs.chromium.org/chromium/src/storage/browser/blob/blob_memory_controller.cc?q=CalculateBlobStorageLimitsImpl&sq=package:chromium).
+
+**In-Memory Storage Limit**
+
+* If the architecture is x64 and NOT ChromeOS or Android: `2GB`
+* Otherwise: `total_physical_memory / 5`
+
+**Disk Storage Limit**
+
+* If ChromeOS: `disk_size / 2`
+* If Android: `disk_size / 20`
+* Else: `disk_size / 10`
+
+Note: ChromeOS's disk is part of the user partition, which is separate from the
+system partition.
+
+**Minimum Disk Availability**
+
+We limit our disk limit to accomidate a minimum disk availability. The equation
+we use is:
+
+`min_disk_availability = in_memory_limit * 2`
+
+## Example Limits
+
+(All sizes in GB)
+
+| Device | Ram | In-Memory Limit | Disk | Disk Limit | Min Disk Availability |
+| --- | --- | --- | --- | --- | --- |
+| Cast | 0.5 | 0.1 | 0 | 0 | 0 |
+| Android Minimal | 0.5 | 0.1 | 8 | 0.4 | 0.2 |
+| Android Fat | 2 | 0.4 | 32 | 1.5 | 0.8 |
+| CrOS | 2 | 0.4 | 8 | 4 | 0.8 |
+| Desktop 32 | 3 | 0.6 | 500 | 50 | 1.2 |
+| Desktop 64 | 4 | 2 | 500 | 50 | 4 |
+
+# Common Pitfalls
+
+## Creating Large Blobs Too Fast
+
+Creating a lot of blobs, especially if they are very large blobs, can cause
+the renderer memory to grow too fast and result in an OOM on the renderer side.
+This is because the renderer temporarily stores the blob data while it waits
+for the browser to request it. Meanwhile, Javascript can continue executing.
+Transfering the data can take a lot of time if the blob is large enough to save
+it directly to a file, as this means we need to wait for disk operations before
+the renderer can get rid of the data.
+
+## Leaking Blob References
+
+If the blob object in Javascript is kept around, then the data will never be
+cleaned up in the backend. This will unnecessarily us memory, so make sure to
+dereference blob objects if they are no longer needed.
+
+Similarily if a URL is created for a blob, this will keep the blob data around
+until the URL is revoked (and the blob object is dereferenced). However, the
+URL is automatically revoked when the browser context is destroyed.
+
+# How to use Blobs (Browser-side)
+
+## Building
+All blob interaction should go through the `BlobStorageContext`. Blobs are
+built using a `BlobDataBuilder` to populate the data and then calling
+`BlobStorageContext::AddFinishedBlob` or `::BuildBlob`. This returns a
+`BlobDataHandle`, which manages reading, lifetime, and metadata access for the
+new blob.
+
+If you have known data that is not available yet, you can still create the blob
+reference, but see the documentation in `BlobDataBuilder::AppendFuture* or
+::Populate*` methods on the builder, the callback usage on
+`BlobStorageContext::BuildBlob`, and
+`BlobStorageContext::NotifyTransportComplete` to facilitate this construction.
+
+## Accessing / Reading
+
+All blob information should come from the `BlobDataHandle` returned on
+construction. This handle is cheap to copy. Once all instances of handles for
+a blob are destructed, the blob is destroyed.
+
+`BlobDataHandle::RunOnConstructionComplete` will notify you when the blob is
+constructed or broken (construction failed due to not enough space, filesystem
+error, etc).
+
+The `BlobReader` class is for reading blobs, and is accessible off of the
+`BlobDataHandle` at any time.
+
+# Blob Creation & Transportation (Renderer)
+
+**This process is outlined with diagrams and illustrations [here](
+https://docs.google.com/presentation/d/1MOm-8kacXAon1L2tF6VthesNjXgx0fp5AP17L7XDPSM/edit#slide=id.g75c319281_0_681).**
+
+This outlines the renderer-side responsabilities of the blob system. The
+renderer needs to:
+
+ 1. Consolidate small bytes items into larger chunks (avoiding a huge array of
+ 1 byte items).
+ 2. Communicate the blob description to the browser immediately on
+ construction.
+ 3. Populate shared memory or files sent from the browser with the consolidated
+ blob data items.
+ 4. Hold the blob data until the browser is finished requesting it.
+
+The meat of blob construction starts in the [WebBlobRegistryImpl](
+https://cs.chromium.org/chromium/src/content/child/blob_storage/webblobregistry_impl.h)'s
+`createBuilder(uuid, content_type)`.
+
+## Blob Data Consolidation
+
+Since blobs are often constructed with arrays with single bytes, we try to
+consolidate all **adjacent** memory blob items into one. This is done in
+[BlobConsolidation](https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_consolidation.h).
+The implementation doesn't actually do any copying or allocating of new memory
+buffers, instead it facilitates the transformation between the 'consolidated'
+blob items and the underlying bytes items. This way we don't waste any memory.
+
+## Blob Transportation (Renderer)
+
+After the blob has been 'consolidated', it is given to the
+[BlobTransportController](https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_transport_controller.h).
+This class:
+
+1. Immediately communicates the blob description to the Browser. We also
+[optimistically send](https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_transport_controller.cc?l=325)
+the blob data if the total memory is less than our IPC threshold.
+2. Stores the blob consolidation for data requests from the browser.
+3. Answers requests from the browser to populate or send the blob data. The
+browser can request the renderer:
+  1. Send items and populate the data in IPC ([code](
+https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_transport_controller.cc?q="case+IPCBlobItemRequestStrategy::IPC")).
+  2. Populate items in shared memory and notify the browser when population is
+complete ([code](https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_transport_controller.cc?q="case+IPCBlobItemRequestStrategy::SHARED_MEMORY")).
+  3. Populate items in files and notify the browser when population is complete
+([code](https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_transport_controller.cc?q="case+IPCBlobItemRequestStrategy::FILE")).
+4. Destroys the blob consolidation when the browser says it's done.
+
+The transport controller also tries to keep the renderer alive while we are
+sending blobs, as if the renderer is closed then we would lose any pending blob
+data. It does this the [incrementing and decrementing the process reference
+count](https://cs.chromium.org/chromium/src/content/child/blob_storage/blob_transport_controller.cc?l=62),
+which should prevent fast shutdown.
+
+# Blob Transportation & Storage (Browser)
+
+The browser side is a little more complicated. We are thinking about:
+
+1. Do we have enough space for this blob?
+2. Pick transportation strategy for blob's components.
+3. Is there enough free memory to transport the blob right now? Or does older
+blob data to be paged to disk first?
+4. Do I need to wait for files to be created?
+5. Do I need to wait for dependent blobs?
+
+## Summary
+
+We follow this general flow for constructing a blob on the browser side:
+
+1. Does the blob fit, and what transportation strategy should be used.
+2. Create our browser-side representation of the blob data, including the data
+items from dependent blobs. We try to share items as much as possible to save
+memory, and allow for the dependent blob items to be not populated yet.
+3. Request memory and/or file quota from the BlobMemoryController, which
+manages our blob storage limits. Quota is necessary for both transportation and
+any copies we have to do from dependent blobs.
+4. If transporation quota is needed and when it is granted:
+  1. Tell the BlobTransportHost to start asking for blob data given the earlier
+  decision of strategy.
+    * The BlobTransportHost populates the browser-side blob data item.
+  2. When transportation is done we notify the BlobStorageContext
+5. When transportation is done, copy quota is granted, and dependent blobs are
+complete, we finish the blob.
+  1. We perform any pending copies from dependent blobs
+  2. We notify any listeners that the blob has been completed.
+
+Note: The transportation sections (steps 1, 2, 3) of this process are described
+(without accounting for blob dependencies) with diagrams and details in [this
+presentation](https://docs.google.com/presentation/d/1MOm-8kacXAon1L2tF6VthesNjXgx0fp5AP17L7XDPSM/edit#slide=id.g75d5729ce_0_105).
+
+## BlobTransportHost
+
+The `BlobTransportHost` is in charge of the actual transportation of the data
+from the renderer to the browser. When the initial description of the blob is
+sent to the browser, the BlobTransportHost asks the BlobMemoryController which
+strategy (IPC, Shared Memory, or File) it should use to transport the file.
+Based on this strategy it can translate the memory items sent from the renderer
+into a browser represetation to facilitate the transportation. See [this](
+https://docs.google.com/presentation/d/1MOm-8kacXAon1L2tF6VthesNjXgx0fp5AP17L7XDPSM/edit#slide=id.g75d5729ce_0_145)
+slide, which illustrates how the browser might segment or split up the
+renderer's memory into transportable chunks.
+
+Once the transport host decides its strategy, it will create its own transport
+state for the blob, including a `BlobDataBuilder` using the transport's data
+segment representation. Then it will tell the `BlobStorageContext` that it is
+ready to build the blob.
+
+When the `BlobStorageContext` tells the transport host that it is ready to
+transport the blob data, the transport host requests all of the data from the
+renderer, populates the data in the `BlobDataBuilder`, and then signals the
+storage context that it is done.
+
+## BlobStorageContext
+
+The `BlobStorageContext` is the hub of the blob storage system. It is
+responsible for creating & managing all the state of constructing blobs, as
+well as all blob handle generation and general blob status access.
+
+When a `BlobDataBuilder` is given to the context, whether from the
+`BlobTransportHost` or from elsewhere, the context will do the following:
+
+1. Find all dependent blobs in the new blob (any blob reference in the blob
+item list), and create a 'slice' of their items for the new blob.
+2. Create the final blob item list representation, which creates a new blob
+item list which inserts these 'slice' items into the blob reference spots. This
+is 'flattening' the blob.
+3. Ask the `BlobMemoryManager` for file or memory quota for the transportation
+if necessary
+  * When the quota request is granted, notify the `BlobTransportHost` that to
+  begin transporting the data.
+4. Ask the `BlobMemoryManager` for memory quota for any copies necessary for
+blob slicing.
+5. Adds completion callbacks to any blobs our blob depends on.
+
+When all of the following conditions are met:
+
+1. The `BlobTransportHost` tells us it has transported all the data (or we
+don't need to transport data),
+2. The `BlobMemoryManager` approves our memory quota for slice copies (or we
+don't need slice copies), and
+3. All dependent blobs are completed (or we don't have dependent blobs),
+
+The blob can finish constructing, where any pending blob slice copies are
+performed, and we set the status of the blob.
+
+### BlobStatus lifecycle
+
+The BlobStatus tracks the construction procedure (specifically the transport
+process), and the copy memory quota and dependent blob process is encompassed
+in `PENDING_INTERNALS`.
+
+Once a blob is finished constructing, the status is set to `DONE` or any of
+the `ERR_*` values.
+
+### BlobSlice
+
+During construction, slices are created for dependent blobs using the given
+offset and size of the reference. This slice consists of the relevant blob
+items, and metadata about possible copies from either end. If blob items can
+entirely be used by the new blob, then we just share the item between the. But
+if there is a 'slice' of the first or last item, then our resulting BlobSlice
+representation will create a new bytes item for the new blob, and store
+necessary copy data for later.
+
+### BlobFlattener
+
+The `BlobFlattener` takes the new blob description (including blob references),
+creates blob slices for all the referenced blobs, and constructs a 'flat'
+representation of the new blob, where all blob references are replaced with the
+`BlobSlice` items. It also stores any copy data from the slices.
+
+## BlobMemoryController
+
+The `BlobMemoryController` is responsable for:
+
+1. Determining storage quota limits for files and memory, including restricting
+file quota when disk space is low.
+2. Determining whether a blob can fit and the transportation strategy to use.
+3. Tracking memory quota.
+4. Tracking file quota and creating files.
+5. Accumulating and evicting old blob data to files to disk.
+