Galaxis Core

Apollo-inspired backend-agnostic fetching library with full SSR support.

This package contains core Galaxis functionality, which is dependency-free, framework-agnostic and as unopinionated as possible.

Installation

yarn add @galaxis/core

Note that you should use a framework-specific wrapper (such as Galaxis React or Galaxis Vue) and not install Galaxis Core directly.

The library is compiled to modern JS, but it should work in all reasonable browsers with the help of properly configured Babel.

Your client environment has to have AbortController. You might need to polyfill it.

Features

Queries

Queries are requests that do not change the system state. They are described as Query objects. Note that some options are ignored in the contexts where they are not applicable.

You can work with queries manually via client.query(), client.fetchQuery(), client.readQuery() and client.watchQuery(), or you can create an ObservableQuery. Note that observable queries should be hidden behind some framework-specific wrapper. The library provides useQuery for React and useQuery for Vue.

A query execution may or may not result in returning the state (data and error) from the cache and/or performing a network request, depending on the fetchPolicy option and the cache state.

A query can also be (re)fetched from the network, which always leads to a network request.

Mutations

Mutations are requests that do change the system state. They are described as Mutation objects.

You can work with mutations manually via client.mutate(), or you can create an ObservableMutation. Note that observable mutations should be hidden behind some framework-specific wrapper. The library provides useMutation for React and useMutation for Vue.

A mutation execution always leads to a network request.

Shared Cache

Queries and mutations work with the same shared Cache. That means that a query can use the data (and/or error!) that was fetched by another query, and query data can be indirectly updated by a mutation. The library provides InMemoryCache as a recommended cache.

Cache usage is specified by the fetchPolicy option. You can opt-out of caching by using fetchPolicy: 'no-cache'. Note that if fetchPolicy is not 'no-cache', you also need to specify the toCache() and, in case of queries, fromCache() options, or the caching will appear broken.

Query errors are automatically cached by the network request id depending on the fetchPolicy.

Mutation errors are never cached.

Optimistic Responses

You can specify optimisticData for mutations. During mutation execution, the cache will immediately be updated with this data, and then with the real data when it arrives. In case of an error, the cache will be reverted to a state without the optimistic data, but with all subsequent changes.

As expected, fetchPolicy: 'no-cache' disables optimistic response.

Query Merging

The library encourages executing queries at arbitrary parts of code and points in time. This way, any component of the application can express its data requirements in isolation from other components.

That would lead to duplicate network requests to the same resources. To prevent that, the library uses query merging.

Query merging means that if a query execution (or fetching) results in performing a network request, the library will reuse an ongoing network request with the same id, if there is one. Network request id is calculated by the client.requestId() function.

The cache will also be updated by all queries, which enables different cache updates for queries with the same network request id. It's discouraged, but not strictly forbidden.

You can't really opt-out of query merging, but you can use the forceRequestOnMerge option of Query to force a new network request (essentially re-run the existing one) if this query is being merged with another. This means that a promise representing a network request may hide more than one actual network request behind it.

Request Queueing

Since queries (and mutations!) are executed at arbitrary points in time, there must be a way to prevent overwriting newer data by the older one. This is done by request queueing.

Request queueing ensures that all queries, which were started before some mutation, are finished before that mutation is started (in no particular order). It also ensures that the next mutation will be started only after the previous one was finished.

A typical queue may look like this:

[a bunch of queries] -> [a mutation] -> [a mutation] -> [a bunch of queries]

Note that request queueing means that a promise representing a network request may hide an actual network request that wasn't started yet. Such a request can still be aborted at any time, regardless of its position in the queue.

Race Conditions Handling

Query merging and request queueing, alongside other techniques, make sure that there are no race conditions, as long as data from queries with different network request ids does not overlap. Even if it does, you are still fine, if you only change the system state by mutations, since you don't really care in which order the same data arrives.

Otherwise, beware that you have a razor-thin chance of overwriting an up-to-date data with an outdated one, if you execute such queries simultaneously. In the future, the library may provide means for relative queueing of such queries.

Full Server-Side Rendering Support

The library is built with SSR in mind. Observable queries can be executed on the server side, and, assuming they are wrapped in a framework-specific wrapper, there is no SSR-specific code in the application components, so they are SSR-ready by default. Note that you can disable fetching on the server by the disableSsr option of Query.

The server waits until all network requests of observable queries are finished and the cache is filled with data and errors, then renders the app based on the cache, and sends resulting HTML with embedded cache to the client.

The exact server rendering logic is framework-specific. The library provides getDataFromTree for React and relies on the built-in SSR functionality of Vue.

Hydrate Stage Optimization

If you're doing SSR, you're going to have a hydrate stage on the client, which is the initial render with cached data. By default, queries with fetchPolicy: 'cache-and-network' will be fetched during the hydrate stage. This is likely undesirable because these requests were just performed on the server.

It can be fixed by setting optimizeOnHydrate: true for all queries by default. In general, you should always do that, unless your cache is not coming from just-performed requests (e.g. you're caching the SSR results, or not doing SSR, but persist the cache to local storage, etc.).

Note that you have to indicate that the hydrate stage is complete by calling client.onHydrateComplete(). The framework-specific providers for React (ClientProvider) and Vue (useClientProvider) do this automatically by default.

High Customizability

The library is completely unopinionated about the network level. You can use fetch, axios, XMLHttpRequest, or any other solution. You can add network requests logging, retries, or timeouts. The library doesn't care in which format your data arrives. Just provide the request option of Request that will abstract it all away. The library provides Fetch as a recommended network interface.

The library is also unopinionated about Cache internals. You can add cache invalidation or persistence, partial or complete. You even should be able to integrate the cache with your own state management solution, should you need so. The library provides InMemoryCache as a recommended cache.

Note that almost everything is configurable on a per-request level. For instance, you can use different network interfaces for different queries!

Caveats / known issues

Cache data is decoupled from requests

By desing, the cache data, unlike errors, is decoupled from actual queries and mutations that update it. It means that when some query data is updated by another query or mutation, the corresponding error can't and won't be cleared. It can lead to funny situations.

For instance, suppose there is a query Q that queries some entity that's initially absent, and there is a mutation M that creates this entity. Q gets executed, which results in a 404 error. Then M is executed, which results in the entity creation. The cache is updated, and now Q has both the newly created entity and the existing 404 error.

Without battle testing it's not clear whether it's a fundamental flaw or a minor quirk. Maybe it can be fixed on the observable queries level by ignoring data updates while there is an error.

Optimistic data is treated as usual data

There is no way to distinguish optimistic data from real data. This leads to several issues:

• Optimistic data will prevent a query with fetchPolicy: 'cache-first' from performing a network request. This one can actually be fixed, because the cache can be updated to provide the real data alongside the optimistic one.
• Optimistic data may "stuck" in observable queries just like real data. It's likely impossible to fix without coupling the data with corresponding queries and mutations. It actually happens in Apollo, so it probably should stay as it is.

Public API

⚠ Anything that is not documented here is not considered a part of public API and may change at any time.

Client

Client is the heart of the library that does all the heavy-lifting.

const client = new Client({
    cache: new MyCache(),
    requestId,
    defaultRequest,
    defaultQuery,
    defaultMutation,
});

Arguments

ClientOptions

⚠ Note that defaults are static for the given client instance. Dynamic defaults would add too much complexity. If you really need dynamic defaults, such as a user-specific header that is common for all requests, you should do it on the network level, somewhere inside the request option of the Request. Ideally, if we're talking about authorization, you should rely on a HttpOnly cookie set by the server.

Name	Type	Description	Required
cache	TCache	A cache for storing normalized data and errors. The library provides InMemoryCache that should work in a lot of cases.	Yes
requestId	(resource: TBaseResource) => string	A function for hashing the resource field of Request. The resulting hash is considered the network request id. The library provides objectHash() that should work in a lot of cases. If you only use the Fetch network interface, consider its requestId() for a bit more human-readable hash.	Yes
defaultRequest	Partial<Request>	Default request. Can't be changed later. Merged shallowly.	No
defaultQuery	Partial<Query>	Default query. Can't be changed later. Merged shallowly.	No
defaultMutation	Partial<Mutation>	Default mutation. Can't be changed later. Merged shallowly.	No

client.query()

Execute the query and optionally subscribe to the changes in its state.

const [queryState, request, unsubscribe] = client.query(query, onChange);

Arguments

Name	Type	Description	Required
query	Query	A query to execute.	Yes
onChange	(state: QueryState) => void	A callback to call when the state of the query changes.	No

Return value

Name	Type	Description
queryState	QueryState	The state of the given query.
request	Promise<TData> | undefined	A promise representing network request. It will be undefined, if it wasn't required (or was required, but wasn't allowed on the server side). Internally, there may be more than one actual network request.
unsubscribe	(() => void) | undefined	A function for unsubscribing from query state updates. Will be undefined if no onChange callback was passed, or if the query itself is not cacheable.

client.fetchQuery()

Fetch the query. This method will always create a network request.

const result = client.fetchQuery(query);

Arguments

Name	Type	Description	Required
query	Query	A query to fetch.	Yes

Return value

Promise<TData>

client.readQuery()

Get state of the given query.

const queryState = client.readQuery(query);

Arguments

Name	Type	Description	Required
query	Query	A query to read.	Yes

Return value

QueryState

Name	Type	Description
data	TData | undefined	Data from the cache. undefined means no data. An unsuccessful request will not overwrite this field. Can be updated by other query or mutation. It can be thought of as the last known data. Always undefined for non-cacheable query.
error	TError | undefined	Error from the cache. undefined means no error. A successful request will overwrite this field to undefined. It can be thought of as the error from the last request. Always undefined for non-cacheable query.
willFetch	boolean	Specifies whether the query will be fetched upon its execution. Note that the fetching can still be disabled on the server side.

client.watchQuery()

Get state of the given query and subscribe to its changes.

const [queryState, unsubscribe] = client.watchQuery(query, onChange);

Arguments

Name	Type	Description	Required
query	Query	A query to read.	Yes
onChange	(state: QueryState) => void	A callback to call when the state of the query changes.	Yes

Return value

Name	Type	Description
queryState	QueryState	The state of the given query.
unsubscribe	() => void | undefined	A function for unsubscribing. Will be undefined if the query is not cacheable.

client.mutate()

Execute the mutation.

const mutationResult = client.mutate(mutation);

Arguments

Name	Type	Description	Required
mutation	Mutation	A mutation to execute.	Yes

Return value

Promise<TData>

client.reset()

Reset the client. Specifically:

• Clear the cache (reset it to empty state).
• Rerun all ongoing network requests of queries.
• Ignore results of ongoing network requests of mutations.
• Notify the subscribers that there was a reset.

You should call this method right after logout.

If you only want to invalidate all cached data, you can call client.getCache().clear().

client.reset();

client.onReset()

Subscribe to client reset.

const unsubscribe = client.onReset(onReset);

Arguments

Name	Type	Description	Required
onReset	() => void	A callback to call on client reset.	Yes

Return value

() => void

client.getCache()

Get cache that was passed to the constructor.

const cache = client.getCache();

Return value

TCache

client.onHydrateComplete()

Report to the client that the hydrate stage is complete. The client always starts in the hydrate stage, and it's a one-way operation.

client.onHydrateComplete();

client.requestId()

Get hash of the given resource using the requestId function that was passed to the constructor.

const requestId = client.requestId(request.resource);

Arguments

Name	Type	Description	Required
resource	TBaseResource	A resource to hash.	Yes

Return value

string

ObservableQuery

ObservableQuery maintains the state of the corresponding query. It has its own state, which synchronizes with the global cache.

ObservableQuery can be active or inactive. If it's inactive, it won't report about its state changes, and the only way to change its state is to call the observableQuery.setOptions() function.

Upon the client reset, all active observable queries will be re-executed. Inactive queries are supposed to be activated shortly (and executed anyway) or disposed.

To make the UI less jumpy, the state synchronization is intentionally not perfect. The following rules apply:

• In loading state all updates are paused, except for updates that happen because of the observableQuery.setOptions() call (or the query execution).
• In the following conditions, If there is no data to show, the previous data is used, and if there is also no error, the previous error is used:
  • Upon switching to loading state (and later). Rationale: don't make significant UI changes for a split second after which a similar data will arrive. If it doesn't arrive, it's still better to show the previous data and the new error. However, if there is no loading, it's better to show the new state right away, since it won't change automatically.
  • Upon cache invalidation (update to empty data when the query is already executed). Rationale: cache invalidation can happen at arbitrary time, and the UI shouldn't suddenly become empty and/or show a dozen loaders.
• Upon the client reset, the previous state will still be shown during the loading state (but will be cleared right after). Rationale: again, don't make significant UI changes for a split second, the data will likely remain the same.

const observableQuery = new ObservableQuery(onChange);

Arguments

Name	Type	Description	Required
onChange	() => void	A function that will be called on observable query state changes.	Yes

observableQuery.setOptions()

Switch the observable query into the inactive state and update its client and query. It may lead to the state change, which won't be reported. You should manually request the state via the observableQuery.getState() function.

Such behavior allows you to update the state and synchronously get it back without worrying about unnecessary calls of the onChange function.

observableQuery.setOptions(client, query);

Arguments

Name	Type	Description	Required
client	Client	A Client instance.	Yes
query	Query	A query to maintain. If no query is passed, the observable query is essentially paused.	No

observableQuery.start()

Activate the observable query. Internally, the provided query is executed, and the observable query starts watching its state. If the query state changed since the last observableQuery.setOptions() call, the observable query state will be updated and immediately reported.

This method is supposed to be called asynchronously after the observableQuery.setOptions() call. This allows to decouple the side effect of query execution from the render phase.

This method is also supposed to be used to wait for the query execution during SSR.

const request = observableQuery.start();

Return value

The request from the query execution, if any.

Promise<TData> | undefined

observableQuery.dispose()

When you don't need the instance anymore, call this method to perform the internal cleanup. Once the observable query is disposed, it can't be used again.

observableQuery.dispose();

observableQuery.getState()

Get the observable query state.

const observableQueryState = observableQuery.getState();

Return value

ObservableQueryState

Name	Type	Description
data	TData | undefined	The last known data. May show previous data from other query, if there is no data for the current query.
error	TError | undefined	The error from the last request. May show previous error from other query, if there is no data and error for the current query.
loading	boolean	Whether the query is being fetched (or will be fetched upon activation) by this observable query.

observableQuery.refetch()

Refetch the query from network. The query has to be executed first.

const result = observableQuery.refetch();

Return value

Promise<TData>

ObservableMutation

ObservableMutation maintains the state of the corresponding mutation. It has its own state.

Contrary to ObservableQuery, ObservableMutation is always active, and the only way to change its state is to call the observableMutation.execute() or the observableMutation.reset() methods.

Upon the client reset, all observable mutations are reset (essentially by calling the observableMutation.reset() method).

Unlike ObservableQuery, ObservableMutation will never show any previous state.

const observableMutation = new ObservableMutation(onChange);

Arguments

Name	Type	Description	Required
onChange	() => void	A function that will be called on observable mutation state changes.	Yes

observableQuery.setOptions()

Update client and mutation of the observable mutation. The state won't be updated.

observableMutation.setOptions(client, mutation);

Arguments

Name	Type	Description	Required
client	Client	A Client instance.	Yes
mutation	Mutation	A mutation to maintain. If no mutation is passed, it has to be specified during the observableMutation.execute() call.	No

observableMutation.execute()

Execute the passed or stored mutation.

const result = observableMutation.execute(mutation);

Arguments

Name	Type	Description	Required
mutation	Mutation	A mutation to execute instead of the stored one.	No

Return value

Promise<TData>

observableMutation.dispose()

When you don't need the instance anymore, call this method to perform the internal cleanup. Once the observable mutation is disposed, it can't be used again.

observableMutation.dispose();

observableMutation.getState()

Get the observable mutation state.

const observableMutationState = observableMutation.getState();

Return value

ObservableMutationState

Name	Type	Description
data	TData | undefined	The data of the last mutation that was executed. Never shows the previous data.
error	TError | undefined	The error of the last mutation that was executed. Never shows the previous error.
loading	boolean	Whether the mutation is being executed.
called	boolean	Whether the mutation was executed. Can only be switched back to false by the observableMutation.reset() call.

observableMutation.reset()

Reset the observable mutation state.

observableMutation.reset();

Important Types

User-defined types

Name	Scope	Constraint	Description
TCache	Client-specific	Must extend Cache	Cache for storing normalized data and errors.
TCacheData	Client-specific	Must extend NonUndefined	Cache data. Normalized data from all requests.
TBaseData	Client-specific	Must extend NonUndefined	Query or mutation data, common for all requests. Used for defaults.
TBaseError	Client-specific	Must extend Error	Query or mutation error, common for all requests. Used for defaults.
TBaseResource	Client-specific	Must extend Resource	Query or mutation resource, common for all requests. Used for defaults.
TData	Request-specific	Must extend TBaseData	Query or mutation data.
TError	Request-specific	Must extend TBaseError	Query or mutation error.
TResource	Request-specific	Must extend TBaseResource	Query or mutation resource, usually the request parameters.

NonUndefined

Anything but undefined.

Resource

The constraint for request resource.

Name	Type	Description	Required
name	string	A globally unique identifier of the resource or the group of similar resources (e.g. /entities/:id). In the latter case, the final resource object must include additional fields to uniquely identify the given resource (e.g. id: 1)	Yes

Request

This type describes base request fields that are common for queries and mutations.

Name	Type	Description	Required
resource	TResource	An object that uniquely describes the given resource. Usually contains request parameters.	Yes
abortSignal	AbortSignal	Signal for aborting the request.	No
request	(resource: TResource, abortSignal?: AbortSignal) => Promise<TData>;	A factory for creating network requests. Note that abortSignal for the factory is created by the library. It is not the same signal as abortSignal field of Request.	No, a rejected promise will be used by default
toCache	(opts: ToCacheOptions) => TCacheData;	A function that modifies the cache data based on the request data (from network or optimistic response). It has to be pure. If you want to update the cache data directly, you can use the immerify() helper.	No, the cache data will not be modified by default.

Query

Extends Request.

Name	Type	Description	Required
fetchPolicy	FetchPolicy	FetchPolicy.	No, 'cache-and-network' is used by default.
disableSsr	boolean	If true, the query will not be fetched on the server.	No
optimizeOnHydrate	boolean	If true, the query won't be fetched on the client during the hydrate stage, if there is data or error in the cache. fetchPolicy option is ignored.	No
forceRequestOnMerge	boolean	If true, the query will start a new network request, if it's merged with the existing query.	No
softAbortSignal	AbortSignal	Soft aborting should be used to indicate loss of interest in the ongoing network request. The actual request won't be aborted if there are other interested parties.	No
fromCache	(opts: FromCacheOptions) => TData | undefined	A function for retrieving the query data from the cache data. It has to be pure, and it has to return the same value if the corresponding cache data haven't changed. If you compute the value, you should use the memoize() helper.	No, a function returning undefined will be used by default

Mutation

Extends Request.

Name	Type	Description	Required
fetchPolicy	FetchPolicy	FetchPolicy.	No, 'cache-and-network' is used by default.
optimisticData	TData	Optimistic data (optimistic response).	No

FromCacheOptions

Name	Type	Description
cacheData	TCacheData	Cache data.
resource	TResource	The resource of the corresponding query/mutation.
requestId	string	Network request id.

ToCacheOptions

Extends FromCacheOptions

Name	Type	Description
data	TData	Data of the given request (from network or optimistic response).

FetchPolicy

fetchPolicy	Query execution result	Mutation execution result
'cache-only'	No network request. Returns state from the cache.	N/A
'cache-first'	Network request, if there is no data in the cache. Returns state from the cache. The cache is then updated with the request result (if there was a network request).	N/A
'cache-and-network'	Network request regardless of the cache state. Returns state from the cache. The cache is then updated with the request result.	Network request regardless of the cache state. The cache data is then updated with the request result.
'no-cache'	Network request regardless of the cache state. Does not touch the cache in any way.	Network request regardless of the cache state. Does not touch the cache in any way.

Cache

Name	Type	Description
subscribe	(callback: () => void) => () => void	Subscribe to the cache. The callback will be called on cache changes. Call returned function to unsubscribe.
update	(opts: UpdateOptions) => void	Update cache state.
getData	() => TCacheData	Get cache data.
getError	(requestId: string) => Error | undefined	Get cached error for the given request id.
clear	() => void	Reset the cache to empty state.

UpdateOptions

Name	Type	Description	Required
data	(prevData: TCacheData) => TCacheData	A function that returns new data based on the previous one.	No
errors	Record<string, (prevError: Error | undefined) => Error | undefined>	A dictionary of errors that need to be updated. The keys are the network request ids, and the values are functions that return new errors based on the previous ones.	No
createSplitFor	NonUndefined	If passed, the cache will split its state (or states in case of multiple splits) in two and apply the updates only to one copy, which will be associated with the passed value.	No
clearSplitFor	NonUndefined	If passed, the cache will drop the corresponding copy of its state, essentially reverting to a state that only lacks the updates introduced by the split, but has all subsequent changes.	No