docs: add v2.1.0 upgrade banner and migration guide to v1 README (#310)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Devesh-Skyflow

README.md

@@ -1,5 +1,7 @@
 # Skyflow Java
 
+> **Java V2.1.0 IS NOW AVAILABLE:** A new, improved version of the Skyflow SDK is ready with flexible authentication, multi-vault support, builder patterns, and richer error diagnostics. V1 is in maintenance mode (security patches only) and will reach End of Life on October 31, 2026. We recommend upgrading to v2.1.0 — see the **[Migration Guide](docs/migrate_to_v2.md)** for step-by-step instructions.
+
 The Skyflow Java SDK is designed to help with integrating Skyflow into a Java backend.
 
 [![CI](https://img.shields.io/static/v1?label=CI&message=passing&color=green?style=plastic&logo=github)](https://github.com/skyflowapi/skyflow-java/actions)

docs/migrate_to_v2.md

@@ -0,0 +1,252 @@
+# Skyflow Java SDK — V1 to V2 Migration Guide
+
+This guide covers the steps to migrate the Skyflow Java SDK from v1 to v2.
+
+---
+
+## Authentication options
+
+In V2, multiple authentication options are available. You can now provide credentials in the following ways:
+
+- Environment variable (`SKYFLOW_CREDENTIALS`) _(Recommended)_
+- API Key
+- Path to credentials JSON file
+- Stringified JSON of credentials
+- Bearer token
+
+**V1 (Old)**
+
+```java
+static class DemoTokenProvider implements TokenProvider {
+    @Override
+    public String getBearerToken() throws Exception {
+        ResponseToken res = null;
+        try {
+            String filePath = "<YOUR_CREDENTIALS_FILE_HERE>";
+            res = Token.generateBearerToken(filePath);
+        } catch (SkyflowException e) {
+            e.printStackTrace();
+        }
+        return res.getAccessToken();
+    }
+}
+```
+
+**V2 (New): Choose one of the following:**
+
+```java
+// Option 1: API Key (Recommended)
+Credentials skyflowCredentials = new Credentials();
+skyflowCredentials.setApiKey("<YOUR_API_KEY>");
+
+// Option 2: Environment Variable (Recommended)
+// Set SKYFLOW_CREDENTIALS in your environment
+
+// Option 3: Credentials File
+skyflowCredentials.setPath("<YOUR_CREDENTIALS_FILE_PATH>");
+
+// Option 4: Stringified JSON
+skyflowCredentials.setCredentialsString("<YOUR_CREDENTIALS_STRING>");
+
+// Option 5: Bearer Token
+skyflowCredentials.setToken("<BEARER_TOKEN>");
+```
+
+> **Notes:**
+> - Use only ONE authentication method per credentials object.
+> - API Key or environment variable are recommended for production.
+> - For priority order see [Quickstart — Initialize the client](../README.md#initialize-the-client).
+
+---
+
+## Initializing the client
+
+V2 introduces a builder pattern for client initialization with multi-vault support.
+
+**Key changes:**
+- `vaultUrl` replaced with `clusterId` (derived from vault URL)
+- Added `env` specification (e.g. `Env.PROD`, `Env.SANDBOX`)
+- Log level is now per-client-instance
+
+**V1 (Old)**
+
+```java
+DemoTokenProvider demoTokenProvider = new DemoTokenProvider();
+SkyflowConfiguration skyflowConfig = new SkyflowConfiguration(
+    "<VAULT_ID>", "<VAULT_URL>", demoTokenProvider
+);
+Skyflow skyflowClient = Skyflow.init(skyflowConfig);
+```
+
+**V2 (New)**
+
+```java
+Credentials credentials = new Credentials();
+credentials.setPath("<YOUR_CREDENTIALS_FILE_PATH>");
+
+VaultConfig config = new VaultConfig();
+config.setVaultId("<YOUR_VAULT_ID>");
+config.setClusterId("<YOUR_CLUSTER_ID>");
+config.setEnv(Env.PROD);
+config.setCredentials(credentials);
+
+Skyflow skyflowClient = Skyflow.builder()
+    .setLogLevel(LogLevel.DEBUG)
+    .addVaultConfig(config)
+    .build();
+```
+
+---
+
+## Request and response structure
+
+V2 removes third-party JSON objects in favour of native `ArrayList` and `HashMap` with a builder pattern for requests.
+
+**V1 (Old) — Request**
+
+```java
+JSONObject recordsJson = new JSONObject();
+JSONArray recordsArrayJson = new JSONArray();
+JSONObject recordJson = new JSONObject();
+recordJson.put("table", "cards");
+JSONObject fieldsJson = new JSONObject();
+fieldsJson.put("cardNumber", "41111111111");
+fieldsJson.put("cvv", "123");
+recordJson.put("fields", fieldsJson);
+recordsArrayJson.add(recordJson);
+recordsJson.put("records", recordsArrayJson);
+try {
+    JSONObject insertResponse = skyflowClient.insert(records);
+} catch (SkyflowException e) {
+    System.out.println(e);
+}
+```
+
+**V2 (New) — Request**
+
+```java
+HashMap<String, Object> value = new HashMap<>();
+value.put("<COLUMN_NAME_1>", "<COLUMN_VALUE_1>");
+value.put("<COLUMN_NAME_2>", "<COLUMN_VALUE_2>");
+ArrayList<HashMap<String, Object>> values = new ArrayList<>();
+values.add(value);
+
+InsertRequest insertRequest = InsertRequest.builder()
+    .table("<TABLE_NAME>")
+    .values(values)
+    .returnTokens(true)
+    .build();
+
+InsertResponse response = skyflowClient.vault().insert(insertRequest);
+```
+
+**V1 (Old) — Response**
+
+```json
+{
+  "records": [
+    {
+      "table": "cards",
+      "fields": {
+        "skyflow_id": "16419435-aa63-4823-aae7-19c6a2d6a19f",
+        "cardNumber": "f3907186-e7e2-466f-91e5-48e12c2bcbc1",
+        "cvv": "1989cb56-63da-4482-a2df-1f74cd0dd1a5"
+      }
+    }
+  ]
+}
+```
+
+**V2 (New) — Response**
+
+```json
+{
+  "insertedFields": [
+    {
+      "skyflowId": "9fac9201-7b8a-4446-93f8-5244e1213bd1",
+      "card_number": "5484-7829-1702-9110",
+      "cardholder_name": "b2308e2a-c1f5-469b-97b7-1f193159399b"
+    }
+  ],
+  "errors": null
+}
+```
+
+---
+
+## Request options
+
+V2 builder pattern replaces V1 options objects.
+
+**V1 (Old)**
+
+```java
+InsertOptions insertOptions = new InsertOptions(true);
+```
+
+**V2 (New)**
+
+```java
+InsertRequest request = InsertRequest.builder()
+    .table("<TABLE_NAME>")
+    .values(values)
+    .continueOnError(false)
+    .tokenMode(TokenMode.DISABLE)
+    .returnTokens(false)
+    .upsert("<UPSERT_COLUMN>")
+    .build();
+```
+
+---
+
+## Error structure
+
+V2 provides richer error details for easier debugging.
+
+**V1 (Old)**
+
+```json
+{
+  "code": "<http_code>",
+  "description": "<description>"
+}
+```
+
+**V2 (New)**
+
+```json
+{
+  "httpStatus": "<http_status>",
+  "grpcCode": "<grpc_code>",
+  "httpCode": "<http_code>",
+  "message": "<message>",
+  "requestId": "<request_id>",
+  "details": ["<details>"]
+}
+```
+
+---
+
+## Credential field names (v2.1+)
+
+The credentials JSON file field names are updated to follow Java camelCase conventions. Both old and new forms are permanently accepted.
+
+| Old form (still accepted) | New form (preferred) |
+|---|---|
+| `clientID` | `clientId` |
+| `keyID` | `keyId` |
+| `tokenURI` | `tokenUri` |
+
+---
+
+## Response field names (v2.1+)
+
+Response maps now return `skyflowId` (camelCase). The legacy `skyflow_id` key is still present for backward compatibility but is deprecated.
+
+| Deprecated (still returned) | Preferred |
+|---|---|
+| `skyflow_id` | `skyflowId` |
+
+---
+
+For the full list of changes see [CHANGELOG.md](../CHANGELOG.md).