// Copyright 2019 The Go Cloud Development Kit Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package docstore provides a portable way of interacting with a document store.
// Subpackages contain driver implementations of docstore for supported
// services.
//
// See https://gocloud.dev/howto/docstore/ for a detailed how-to guide.
//
// # Collections
//
// In docstore, documents are grouped into collections, and each document has a key
// that is unique in its collection. You can add, retrieve, modify and delete
// documents by key, and you can query a collection to retrieve documents that match
// certain criteria.
//
// # Representing Documents
//
// A document is a set of named fields, each with a value. A field's value can be a scalar,
// a list, or a nested document.
//
// Docstore allows you to represent documents as either map[string]interface{} or
// struct pointers. When you represent a document as a map, the fields are map keys
// and the values are map values. Lists are represented with slices. For example,
// here is a document about a book described as a map:
//
//	doc := map[string]interface{}{
//	    "Title": "The Master and Margarita",
//	    "Author": map[string]interface{}{
//	        "First": "Mikhail",
//	        "Last": "Bulgakov",
//	    },
//	    "PublicationYears": []int{1967, 1973},
//	}
//
// Note that the value of "PublicationYears" is a list, and the value of "Author" is
// itself a document.
//
// Here is the same document represented with structs:
//
//	type Book struct {
//	    Title            string
//	    Author           Name
//	    PublicationYears []int
//	}
//
//	type Name struct {
//	    First, Last string
//	}
//
//	doc := &Book{
//	    Title: "The Master and Margarita",
//	    Author: Name{
//	        First: "Mikhail",
//	        Last:  "Bulgakov",
//	    },
//	    PublicationYears: []int{1967, 1973},
//	}
//
// You must use a pointer to a struct to represent a document, although structs
// nested inside a document, like the Name struct above, need not be pointers.
//
// Maps are best for applications where you don't know the structure of the
// documents. Using structs is preferred because it enforces some structure on your
// data.
//
// By default, Docstore treats a struct's exported fields as the fields of the
// document. You can alter this default mapping by using a struct tag beginning
// with "docstore:". Docstore struct tags support renaming, omitting fields
// unconditionally, or omitting them only when they are empty, exactly like
// encoding/json. For example, this is the Book struct with different field
// names:
//
//	type Book struct {
//	    Title            string `docstore:"title"`
//	    Author           Name   `docstore:"author"`
//	    PublicationYears []int  `docstore:"pub_years,omitempty"`
//	    NumPublications  int    `docstore:"-"`
//	}
//
// This struct describes a document with field names "title", "author" and
// "pub_years". The pub_years field is omitted from the stored document if it has
// length zero. The NumPublications field is never stored because it can easily be
// computed from the PublicationYears field.
//
// Given a document field "Foo" and a struct type document, Docstore's decoder
// will look through the destination struct's field to find (in order of
// preference):
//   - An exported field with a tag of "Foo";
//   - An exported field named "Foo".
//
// Note that unlike encoding/json, Docstore does case-sensitive matching during
// decoding to match the behavior of decoders in most docstore services.
//
// # Representing Data
//
// Values stored in document fields can be any of a wide range of types. All
// primitive types except for complex numbers are supported, as well as slices and
// maps (the map key type must be a string, an integer, or a type that implements
// encoding.TextMarshaler). In addition, any type that implements
// encoding.BinaryMarshaler or encoding.TextMarshaler is permitted. This set of types
// closely matches the encoding/json package (see https://golang.org/pkg/encoding/json).
//
// Times deserve special mention. Docstore can store and retrieve values of type
// time.Time, with two caveats. First, the timezone will not be preserved. Second,
// Docstore guarantees only that time.Time values are represented to millisecond
// precision. Many services will do better, but if you need to be sure that times
// are stored with nanosecond precision, convert the time.Time to another type before
// storing and re-create when you retrieve it. For instance, if you store Unix
// time in nanoseconds using time's UnixNano method, you can get the original
// time back (in the local timezone) with the time.Unix function.
//
// # Representing Keys
//
// The key of a docstore document is its unique identifier, usually a field.
// Keys never appear alone in the docstore API, only as part of a document. For
// instance, to retrieve a document by key, you pass the Collection.Get method
// a document as a struct pointer or map with the key field populated, and docstore
// populates the rest of that argument with the stored contents. Docstore
// doesn't take zero-value key.
//
// When you open a collection using an OpenCollection method of the
// service-specific driver or a URL, you specify how to extract the key from a
// document.
// Usually, you provide the name of the key field, as in the example below:
//
//	coll, err := memdocstore.OpenCollection("SSN", nil)
//
// Here, the "SSN" field of the document is used as the key. Some drivers let you
// supply a function to extract the key from the document, which can be useful if the
// key is composed of more than one field.
//
// # Actions
//
// Docstore supports six actions on documents as methods on the Collection type:
//   - Get retrieves a document.
//   - Create creates a new document.
//   - Replace replaces an existing document.
//   - Put puts a document into a collection, replacing it if it is already present.
//   - Update applies a set of modifications to a document.
//   - Delete deletes a document.
//
// Each action acts atomically on a single document. You can execute actions
// individually or you can group them into an action list, like so:
//
//	err := coll.Actions().Put(doc1).Replace(doc2).Get(doc3).Do(ctx)
//
// When you use an action list, docstore will try to optimize the execution of the
// actions. For example, multiple Get actions may be combined into a single "batch
// get" RPC. For the most part, actions in a list execute in an undefined order
// (perhaps concurrently) and independently, but read and write operations on the same
// document are executed in the user-specified order. See the documentation of
// ActionList for details.
//
// # Revisions
//
// Docstore supports document revisions to distinguish different versions of a
// document and enable optimistic locking. By default, Docstore stores the
// revision in the field named "DocstoreRevision" (stored in the constant
// DefaultRevisionField). Providers give you the option of changing that field
// name.
//
// When you pass a document with a revision field to a write action, Docstore
// will give it a revision at creation time or update the revision value when
// modifying the document. If you don't want Docstore to handle any revision
// logic, simply do not have the revision field in your document.
//
// When you pass a document with a non-nil revision to Put, Replace, Update or
// Delete, Docstore will also compare the revision of the stored document to
// that of the given document before making the change. It returns an error with
// code FailedPrecondition on mismatch. (See https://gocloud.dev/gcerrors for
// information about error codes.) If modification methods are called on a
// document struct or map a nil revision field, then no revision checks are
// performed, and changes are forced blindly, but a new revision will still be
// given for the document. For example, if you call Get to retrieve a document
// with a revision, then later perform a write action with that same document,
// it will fail if the document was changed since the Get.
//
// Since different services use different types for revisions, revision fields
// of unspecified type must be handled. When defining a document struct,
// define the field to be of type interface{}. For example,
//
//	type User {
//	    Name             string
//	    DocstoreRevision interface{}
//	}
//
// # Queries
//
// Docstore supports querying within a collection. Call the Query method on
// Collection to obtain a Query value, then build your query by calling Query methods
// like Where, Limit and so on. Finally, call the Get method on the query to execute it.
// The result is an iterator, whose use is described below.
//
//	iter := coll.Query().Where("size", ">", 10).Limit(5).Get(ctx)
//
// The Where method defines a filter condition, much like a WHERE clause in SQL.
// Conditions are of the form "field op value", where field is any document field
// path (including dot-separated paths), op is one of "=", ">", "<", ">=" or "<=",
// and value can be any value.
//
//	iter := coll.Query().Where("Author.Last", "=", "Bulgakov").Limit(3).Get(ctx)
//
// Dot-separated paths work as you'd expect for nested structs and maps, addressing
// the nested struct field or map key. For nested slices, some providers (notably
// MongoDB) treat s.f, where s is a sequence, as the slice of e.f for each e in s.
// However, others do not support field selectors inside slices, so docstore does
// not support them -- while you may find that they work with some providers, you
// cannot rely on that if you need fully portable code.
//
// You can make multiple Where calls. In some cases, parts of a Where clause may be
// processed in the driver rather than natively by the backing service, which may have
// performance implications for large result sets. See the driver package
// documentation for details.
//
// Use the DocumentIterator returned from Query.Get by repeatedly calling its Next
// method until it returns io.EOF. Always call Stop when you are finished with an
// iterator. It is wise to use a defer statement for this.
//
//	iter := coll.Query().Where("size", ">", 10).Limit(5).Get(ctx)
//	defer iter.Stop()
//	for {
//	    m := map[string]interface{}{}
//	    err := iter.Next(ctx, m)
//	    if err == io.EOF {
//	        break
//	    }
//	    if err != nil {
//	        return err
//	    }
//	    fmt.Println(m)
//	}
//
// # Errors
//
// The errors returned from this package can be inspected in several ways:
//
// The Code function from https://gocloud.dev/gcerrors will return an error code, also
// defined in that package, when invoked on an error.
//
// The Collection.ErrorAs method can retrieve the underlying driver error from
// the returned error. See the specific driver's package doc for the supported
// types.
//
// # OpenCensus Integration
//
// OpenCensus supports tracing and metric collection for multiple languages and
// backend providers. See https://opencensus.io.
//
// This API collects OpenCensus traces and metrics for the following methods:
//   - ActionList.Do
//   - Query.Get (for the first query only; drivers may make additional calls while iterating over results)
//
// All trace and metric names begin with the package import path.
// The traces add the method name.
// For example, "gocloud.dev/docstore/ActionList.Do".
// The metrics are "completed_calls", a count of completed method calls by driver,
// method and status (error code); and "latency", a distribution of method latency
// by driver and method.
// For example, "gocloud.dev/docstore/latency".
//
// To enable trace collection in your application, see "Configure Exporter" at
// https://opencensus.io/quickstart/go/tracing.
// To enable metric collection in your application, see "Exporting stats" at
// https://opencensus.io/quickstart/go/metrics.
//
// # GORM-like Code Generation
//
// You may find https://github.com/bartventer/docstore-gen useful.
package docstore // import "gocloud.dev/docstore"
