I have a messaging app, where I have a Chats collection in my Firebase Firestore database. I use a custom object which is Codable to read and write changes to firebase.
struct ChatFirebaseDO: Codable {
#DocumentID var id: String?
... {100 other fields} ...
var lastMessageDate: Date
}
When a user sends a new message, I update this lastMessageDate with the FieldValue.serverTimestamp()
I also have a listener which is listening for changes, and it immediately returns any update to me (wnether that is a new Chat of an update to an existing one). However if it is my own user that has created this new chat, it will be returned to me with a null timestamp.
From the docs I see this is intentional behaviour. It suggests that I change replace the nulls with estimated timestamps values (perfect!) however, I can't work out how to combine this with my custom objects.
To get the estimated timestamps, I need to do this:
diff.document.data(with: .estimate)
which returns a dictionary of fields.
But for my Codable custom objects to work, I have to use:
let messageDO = try diff.document.data(as: ChatFirebaseDO.self)
which uses a document (not a dictionary of data).
Is there a way I can (1) replace the nulls with estimated timestamps but (2) still have a document object I can use for my custom object transformation.
Perhaps its a global setting I can make to use estimates, or locally within a single listener request. Or perhaps it is a way to use custom objects from a data dictionary and not just from the FIRDocument.
Thank you in advance!
If you're not encoding these chats for disk storage then why are they even Codable is a question to ask yourself. This particular method is made for that purpose so I'd argue you're using the wrong tool for the job—a tool that also doesn't work because of the timestamp conflict, which I imagine will be addressed in a future update to Firestore.
That said, the timestamps (which are tokens) only return nil when they haven't reached the server which means only from latency-compensated snapshots generated by the client (or only when the signed-in user posts). Therefore, you can provide your own estimate when the value is nil (which would be the current date and time), which would not only be accurate but would be overwritten by the subsequent snapshot anyway when it has a real value. It's not a pleasant workaround but it accomplishes exactly what the token does with its own estimate.
If you don't want to ditch Codable then you can ditch Firestore's timestamp, which I've personally done. I'm not a fan of the token system and I've replaced it with a basic Unix Timestamp (an integer) that makes things much simpler. I don't have to worry about nil times, latency-compensated returns, or configuring snapshot data just to handle the value of a single field. If I had to guess, I would imagine Firestore will eventually allow a global setting of timestamp behavior in addition to expanding the API to allow the Codable method to also account for timestamp behavior. The TLDR of it is that what you want doesn't yet exist natively in the Firestore SDK, unfortunately, and I'd consider making it a feature request on the Firestore-iOS git repo.
Related
Trying to implement Event Sourcing and CQRS for the first time, but got stuck when it came to persisting the aggregates.
This is where I'm at now
I've setup "EventStore" an a stream, "foos"
Connected to it from node-eventstore-client
I subscribe to events with catchup
This is all working fine.
With the help of the eventAppeared event handler function I can build the aggregate, whenever events occur. This is great, but what do I do with it?
Let's say I build and aggregate that is a list of Foos
[
{
id: 'some aggregate uuidv5 made from barId and bazId',
barId: 'qwe',
bazId: 'rty',
isActive: true,
history: [
{
id: 'some event uuid',
data: {
isActive: true,
},
timestamp: 123456788,
eventType: 'IsActiveUpdated'
}
{
id: 'some event uuid',
data: {
barId: 'qwe',
bazId: 'rty',
},
timestamp: 123456789,
eventType: 'FooCreated'
}
]
}
]
To follow CQRS I will build the above aggregate within a Read Model, right? But how do I store this aggregate in a database?
I guess just a nosql database should be fine for this, but I definitely need a db since I will put a gRPC APi in front of this and other read models / aggreates.
But what do I actually go from when I have built the aggregate, to when to persist it in the db?
I once tried following this tutorial https://blog.insiderattack.net/implementing-event-sourcing-and-cqrs-pattern-with-mongodb-66991e7b72be which was super simple, since you'd use mongodb both as the event store and just create a view for the aggregate and update that one when new events are incoming. It had it's flaws and limitations (the aggregation pipeline) which is why I now turned to "EventStore" for the event store part.
But how to persist the aggregate, which is currently just built and stored in code/memory from events in "EventStore"...?
I feel this may be a silly question but do I have to loop over each item in the array and insert each item in the db table/collection or do you somehow have a way to dump the whole array/aggregate there at once?
What happens after? Do you create a materialized view per aggregate and query against that?
I'm open to picking the best db for this, whether that is postgres/other rdbms, mongodb, cassandra, redis, table storage etc.
Last question. For now I'm just using a single stream "foos", but at this level I expect new events to happen quite frequently (every couple of seconds or so) but as I understand it you'd still persist it and update it using materialized views right?
So given that barId and bazId in combination can be used for grouping events, instead of a single stream I'd think more specialized streams such as foos-barId-bazId would be the way to go, to try and reduce the frequency of incoming new events to a point where recreating materialized views will make sense.
Is there a general rule of thumb saying not to recreate/update/refresh materialized views if the update frequency gets below a certain limit? Then the only other a lternative would be querying from a normal table/collection?
Edit:
In the end I'm trying to make a gRPC api that has just 2 rpcs - one for getting a single foo by id and one for getting all foos (with optional field for filtering by status - but that is not so important). The simplified proto would look something like this:
rpc GetFoo(FooRequest) returns (Foo)
rpc GetFoos(FoosRequest) returns (FooResponse)
message FooRequest {
string id = 1; // uuid
}
// If the optional status field is not specified, return all foos
message FoosRequest {
// If this field is specified only return the Foos that has isActive true or false
FooStatus status = 1;
enum FooStatus {
UNKNOWN = 0;
ACTIVE = 1;
INACTIVE = 2;
}
}
message FoosResponse {
repeated Foo foos;
}
message Foo {
string id = 1; // uuid
string bar_id = 2 // uuid
string baz_id = 3 // uuid
boolean is_active = 4;
repeated Event history = 5;
google.protobuf.Timestamp last_updated = 6;
}
message Event {
string id = 1; // uuid
google.protobuf.Any data = 2;
google.protobuf.Timestamp timestamp = 3;
string eventType = 4;
}
The incoming events would look something like this:
{
id: 'some event uuid',
barId: 'qwe',
bazId: 'rty',
timestamp: 123456789,
eventType: 'FooCreated'
}
{
id: 'some event uuid',
isActive: true,
timestamp: 123456788,
eventType: 'IsActiveUpdated'
}
As you can see there is no uuid to make it possible to GetFoo(uuid) in the gRPC API, which is why I'll generate a uuidv5 with the barId and bazId, which will combined, be a valid uuid. I'm making that in the projection / aggregate you see above.
Also the GetFoos rpc will either return all foos (if status field is left undefined), or alternatively it'll return the foo's that has isActive that matches the status field (if specified).
Yet I can't figure out how to continue from the catchup subscription handler.
I have the events stored in "EventStore" (https://eventstore.com/), using a subscription with catchup, I have built an aggregate/projection with an array of Foo's in the form that I want them, but to be able to get a single Foo by id from a gRPC API of mine, I guess I'll need to store this entire aggregate/projection in a database of some sort, so I can connect and fetch the data from the gRPC API? And every time a new event comes in I'll need to add that event to the database also or how is this working?
I think I've read every resource I can possibly find on the internet, but still I'm missing some key pieces of information to figure this out.
The gRPC is not so important. It could be REST I guess, but my big question is how to make the aggregated/projected data available to the API service (possible more API's will need it as well)? I guess I will need to store the aggregated/projected data with the generated uuid and history fields in a database to be able to fetch it by uuid from the API service, but what database and how is this storing process done, from the catchup event handler where I build the aggregate?
I know exactly how you feel! This is basically what happened to me when I first tried to do CQRS and ES.
I think you have a couple of gaps in your knowledge which I'm sure you will rapidly plug. You hydrate an aggregate from the event stream as you are doing. That IS your aggregate persisted. The read model is something different. Let me explain...
Your read model is the thing you use to run queries against and to provide data for display to a UI for example. Your aggregates are not (directly) involved in that. In fact they should be encapsulated. Meaning that you can't 'see' their state from the outside. i.e. no getter and setters with the exception of the aggregate ID which would have a getter.
This article gives you a helpful overview of how it all fits together: CQRS + Event Sourcing – Step by Step
The idea is that when an aggregate changes state it can only do so via an event it generates. You store that event in the event store. That event is also published so that read models can be updated.
Also looking at your aggregate it looks more like a typical read model object or DTO. An aggregate is interested in functionality, not properties. So you would expect to see void public functions for issuing commands to the aggregate. But not public properties like isActive or history.
I hope that makes sense.
EDIT:
Here are some more practical suggestions.
"To follow CQRS I will build the above aggregate within a Read Model, right? "
You do not build aggregates in the read model. They are separate things on separate sides of the CQRS side of the equation. Aggregates are on the command side. Queries are done against read models which are different from aggregates.
Aggregates have public void functions and no getter or setters (with the exception of the aggregate id). They are encapsulated. They generate events when their state changes as a result of a command being issued. These events are stored in an event store and are used to recover the state of an aggregate. In other words, that is how an aggregate is stored.
The events go on to be published so the event handlers and other processes can react to them and update the read model and or trigger new cascading commands.
"Last question. For now I'm just using a single stream "foos", but at this level I expect new events to happen quite frequently (every couple of seconds or so) but as I understand it you'd still persist it and update it using materialized views right?"
Every couple of seconds is very likely to be fine. I'm more concerned at the persist and update using materialised views. I don't know what you mean by that but it doesn't sound like you have the right idea. Views should be very simple read models. No need to complex relations like you find in an RDMS. And is therefore highly optimised fast for reading.
There can be a lot of confusion on all the terminologies and jargon used in DDD and CQRS and ES. I think in this case, the confusion lies in what you think an aggregate is. You mention that you would like to persist your aggregate as a read model. As #Codescribler mentioned, at the sink end of your event stream, there isn't a concept of an aggregate. Concretely, in ES, commands are applied onto aggregates in your domain by loading previous events pertaining to that aggregate, rehydrating the aggregate by folding each previous event onto the aggregate and then applying the command, which generates more events to be persisted in the event store.
Down stream, a subscribing process receives all the events in order and builds a read model based on the events and data contained within. The confusion here is that this read model, at this end, is not an aggregate per se. It might very well look exactly like your aggregate at the domain end or it could be only creating a read model that doesn't use all the events and or the event data.
For example, you may choose to use every bit of information and build a read model that looks exactly like the aggregate hydrated up to the newest event(likely your source of confusion). You may instead have another process that builds a read model that only tallies a specific type of event. You might even subscribe to multiple streams and "join" them into a big read model.
As for how to store it, this is really up to you. It seems to me like you are taking the events and rebuilding your aggregate plus a history of events in a memory structure. This, of course, doesn't scale, which is why you want to store it at rest in a database. I wouldn't use the memory structure, since you would need to do a lot of state diffing when you flush to the database. You should be modify the database directly in response to each individual event. Ideally, you also transactionally store the stream count with said modification so you don't process the same event again in the case of a failure.
Hope this helps a bit.
Transactions in Cloud Firestore support getting a document using transaction.getDocument, but even though there is a .getDocuments method, there doesn’t seem to be a .getDocuments for getting multiple documents that works with a transaction.
I have a Yelp-like app using a Cloud Firestore database with the following structure:
- Places to rate are called spots.
- Each spot has a document in the spots collection (identified by a unique documentID).
- Each spot can have a reviews collection containing all reviews for that spot.
- Each review is identified by its own unique documentID, and each review document contains a rating of the spot.
Below is an image of my Cloud Firestore setup with some data.
I’ve tried to create a transaction getting data for all of the reviews in a spot, with the hope that I could then make an updated calculation of average review & save this back out to a property of the spot document. I've tried using:
let db = Firestore.firestore()
db.runTransaction({ (transaction, errorPointer) -> Any? in
let ref = db.collection("spots").document(self.documentID).collection("reviews")
guard let document = try? transaction.getDocuments(ref) else {
print("*** ERROR trying to get document for ref = \(ref)")
return nil
}
…
Xcode states:
Value of type ‘Transaction’ has no member ‘getDocuments’.
There is a getDocument, which that one can use to get a single document (see https://firebase.google.com/docs/firestore/manage-data/transactions).
Is it possible to get a collection of documents in a transaction? I wanted to do this because each place I'm rating (spot) has an averageRating, and whenever there's a change to one of the ratings, I want to call a function that:
- starts a transaction (done)
- reads in all of the current reviews for that spot (can't get to work)
- calculates the new averageRating
- updates the spot with the new averageRating value.
I know Google's FriendlyEats uses a technique where each change is applied to the current average rating value, but I'd prefer to make a precise re-calculation with each change to keep numerical precision (even if it's a bit more expensive w/an additional query).
Thanks for advice.
No. Client libraries do not allow you to make queries inside of transactions. You can only request specific documents inside of a query. You could do something really hacky, like run the query outside of the transaction, then request every individual document inside the transaction, but I would not recommend that.
What might be better is to run this on the server side. Like, say, with a Cloud Function, which does allow you to run queries inside transactions. More importantly, you no longer have to trust the client to update the average review score for a restaurant, which is a Bad Thing.
That said, I still might recommend using a Cloud Function that does some of the same logic that Friendly Eats does, where you say something along the lines of New average = Old average + new review / (Total number of reviews) It'll make sure you're not performing excessive reads if your app gets really popular.
I am using Sitecore 8.1 with xDB enabled (MongoDB). I would like to store the user-roles of the visiting users in the xDB, so I can aggregate on these data in my reports. These roles can change over time, so one user could have one set of roles at some point in time and another set of roles at a later time.
I could go and store these user-roles as custom facets on the Contact entity, but as they may change for a user from visit to visit, I will loose historical data if I update the data in the facet every time the user log in (fx. I will not be able to tell which roles a given user had, at some given visit).
Instead, I could create a custom IElement for my facet data, and store the roles along with a timestamp saying when the given roles were registered, but this model may be hard to handle during the reporting phase, where I would need to connect the interaction data with the role-data based on timestamps every time I generate a report.
Is it possible to store these custom data in the xDB in something else than the Contact collection? Can I store custom data in the Interactions collection? There is a property called Tracker.Current.Session.Interaction.CustomValues which sounds like what I need, but if I store data here, will I be able to perform proper aggregation/reporting on the data? Any other approaches I haven't thought about?
CustomValues
Yes, the CustomValues dictionary is what I would use in your case. This dictionary will get serialized to MongoDB as a nested document of every interaction (unless the dictionary is empty).
Also note that, since CustomValues is a member of the base class Sitecore.Analytics.Model.Entity, this dictionary is available in many other data classes of xDB. For example, you can store custom values in PageData and PageEventData objects.
Since CustomValues takes an object of any class, your custom data class needs some extra things for it to be successfully saved to and subsequently loaded from MongoDB:
It has to be marked as [Serializable].
It needs to be registered in the MongoDB driver like this:
using Sitecore.Analytics.Data.DataAccess.MongoDb;
// [...]
MongoDbObjectMapper.Instance.RegisterModelExtension<YourCustomClassName>();
This needs to be done only once per application lifetime - for example, in an initialize pipeline processor.
Your own storage
Of course, you don't have to use Sitecore's API to store your custom data. So the alternative would be to manually save data to a custom MongoDB collection or an SQL table. You can then read that data in your aggregation processor, finding it by the ID of currently processed interaction.
The benefit of this approach is that you can decide where and how your data is stored. The downside is extra work of implementing and maintaining this data storage.
I am studying graphql.
I can retrieve data from my mongo database with queries, I can create data with mutations.
But how I can modify existing data?
I am a bit lost here...
I have to create a new mutation?
Yes, every mutation describes a specific action that can be done to a bit of data. GraphQL is not like REST - it doesn't specify any standard CRUD-type actions.
When you are writing a mutation to update some data, you have two options. Let's explain them in the context of a todo item that has a completed status, and a text field:
Write mutations that represent semantic actions - markTodoCompleted, updateTodoText, etc.
Write a generic mutation that just sets any properties passed it, you could call it updateTodo.
I prefer the first approach, because it makes it more clear what the client is doing when it calls a certain mutation. In the second approach, you need to be careful to validate the values to be set to make sure someone can't set some invalid combination.
In short, you need to define your own mutations to update data.
I'm connecting to a third-party web service to retrieve rows from the underlying database. I can optionally pass a parameter like this:
http://server.com/resource?createdAfter=[yyyy-MM-dd hh:ss]
to get only the rows created after a given date.
This means I have to store the current timestamp (using #[function:datestamp:...], no problem) in one message scope and then retrieve it in another.
It also implies the timestamp should be preserved in case of an outage.
Obviously, I could use a subflow containing a file endpoint, saving in a designated file on a path. But, intuitively, based on my (very!) limited experience, it feels hackish.
What's the correct idiom to solve this?
Thanks!
The Object Store Module is designed just for that: to allow you to save bits of information from your flows.
See:
http://mulesoft.github.io/mule-module-objectstore/mule/objectstore-config.html
https://github.com/mulesoft/mule-module-objectstore/