How to make a successful batch commit when some of the docs could've been deleted? - firebase

Question

Suppose i have a structure like this:
items: {
id1: {
likes: 123
},
id2: {
likes: 456
},
id3: {
sourceId: 'id1',
likes: 123
},
id4: {
sourceId: 'id1',
likes: 123
}
[,...]
}
where any item can either be a source item or an item that references a source item. The purpose of referencing the source item is to keep counters consistent across all items that share the same sourceId.
Therefore, when i change a counter on a source item, i want to batch write that counter's value to all the items that have that item as their sourceId.
My concern
Suppose I read in the docs referencing some sourceId, then i commit the changes in a batch to all of them. What if a very small subset of the docs in the batch were deleted in the small window of time since the documents were read in, or a rare but possible write-rate conflict occurs? Do none of the counters get updated because 1 or 2 documents failed to be updated?
Is it possible to submit a batch that writes the changes to each of its documents independently, such that if one fails, it has no impact on if the others get written?
Or maybe for this scenario it might be better to read in the documents referencing some sourceId and then write the changes to each document in parallel such that write independence is achieved. I don't like this approach since the number of requests would be the size of the batch.
What are your thoughts?

Answer 1

Take a careful read of the API docs for BatchWrite. It will answer your questions. Since you're not showing your batch code (are you using set? update?), we have to look at the API docs to assess the failure cases:
create()
This will fail the batch if a document exists at its location.
It sounds like you're probably not using create(), but this is the failure case.
set()
If the document does not exist yet, it will be created.
So, a set will not fail if the documented was deleted before the batch got committed.
update()
If the document doesn't yet exist, the update fails and the entire
batch will be rejected.
So, if you try to update a nonexistent document, the batch will fail.
If you want to decide what to do with each document, depending on its existence and contents, then control the failure cases, use a transaction.

Answer 2

If I understand your question, I had a similar scenario and here's how I did it. First, I use generated universal ID's, uid's, for all my item keys/id's. Then, what you do is on the grandchildren, simply write the uid of the parent it is associated with. Each grandchild could be associated with more than one parent.
As you create new items, you have to recursively update the parent with the uid of the item so the parent has record of all its associated child items.
fetchPromise = [];
fetchArray = [];
if(!selectIsEmpty("order-panel-one-series-select") || !selectIsUnselected(orderPanelOneSeriesSelect)){
orderPanelOneTitle.innerHTML = "Select " + orderPanelOneSeriesSelect.value.toString().toUpperCase() + " Option";
}
//on change we need to populate option select based on this value
//now we need to get the particular series doc and fill get array then prmise all to get all options
familyuid = getRecordFromList(doc.getElementById("order-panel-one-family-select"),orderPanelOneFamilySelect.selectedIndex);
seriesuid = getRecordFromList(doc.getElementById("order-panel-one-series-select"),orderPanelOneSeriesSelect.selectedIndex);
optionuid = getRecordFromList(doc.getElementById("order-panel-one-option-select"),orderPanelOneOptionsSelect.selectedIndex);
optionRef = db.collection("products").doc(familyuid).collection("option");
itemRef = db.collection("products").doc(familyuid).collection("item");
targetRef = db.collection("products").doc(familyuid).collection("option").doc(optionuid);
try {
targetRef.get().then(function(snap) {
if (snap.exists) {
for (var key in snap.data()) {
if (snap.data().hasOwnProperty(key)) {
fetchPromise = itemRef.doc(key).get();
fetchArray.push(fetchPromise);
}
}
Promise.all(fetchArray).then(function(values) {
populateSelectFromQueryValues(values,"order-panel-one-item-select");
if(!selectIsEmpty("order-panel-one-item-select")){
enableSelect("order-panel-one-item-select");
}
targetRef.get().then(function(snap){
if(snap.data().name){
var str = snap.data().name.toString();
orderAsideInfo.innerHTML = "Select " + capitalizeFirstLetter(str) + " item.";
}
});
});
}
}).catch(function(error) {
toast("error check console");
console.log("Error getting document:", error);
});
}