diff --git a/zetaclient/chains/evm/base/observer.go b/zetaclient/chains/evm/base/observer.go
index 29fc185256..1865560fd5 100644
--- a/zetaclient/chains/evm/base/observer.go
+++ b/zetaclient/chains/evm/base/observer.go
@@ -20,6 +20,11 @@ import (
 	clienttypes "github.com/zeta-chain/zetacore/zetaclient/types"
 )
 
+const (
+	// DefaultBlockCacheSize is the default size of the block cache
+	DefaultBlockCacheSize = 1000
+)
+
 // Observer is the base chain observer
 type Observer struct {
 	// the external chain
@@ -63,6 +68,7 @@ func NewObserver(
 	zetacoreContext *context.ZetacoreContext,
 	zetacoreClient interfaces.ZetacoreClient,
 	tss interfaces.TSSSigner,
+	blockCacheSize int,
 	dbPath string,
 	ts *metrics.TelemetryServer,
 ) (*Observer, error) {
@@ -80,15 +86,15 @@ func NewObserver(
 
 	// create block cache
 	var err error
-	ob.blockCache, err = lru.New(1000)
+	ob.blockCache, err = lru.New(blockCacheSize)
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrap(err, "error creating block cache")
 	}
 
 	// open database
 	err = ob.OpenDB(dbPath)
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrap(err, fmt.Sprintf("error opening observer db for chain: %s", chain.ChainName))
 	}
 
 	return &ob, nil
@@ -121,32 +127,44 @@ func (ob *Observer) ZetacoreContext() *context.ZetacoreContext {
 	return ob.zetacoreContext
 }
 
+// ZetacoreClient returns the zetacore client for the observer
+func (ob *Observer) ZetacoreClient() interfaces.ZetacoreClient {
+	return ob.zetacoreClient
+}
+
 // WithZetacoreClient attaches a new zetacore client to the observer
 func (ob *Observer) WithZetacoreClient(client interfaces.ZetacoreClient) *Observer {
 	ob.zetacoreClient = client
 	return ob
 }
 
-// GetLastBlock get external last block height
-func (ob *Observer) GetLastBlock() uint64 {
+// Tss returns the tss signer for the observer
+func (ob *Observer) TSS() interfaces.TSSSigner {
+	return ob.tss
+}
+
+// LastBlock get external last block height
+func (ob *Observer) LastBlock() uint64 {
 	return atomic.LoadUint64(&ob.lastBlock)
 }
 
-// SetLastBlock set external last block height
-func (ob *Observer) SetLastBlock(height uint64) {
-	atomic.StoreUint64(&ob.lastBlock, height)
+// WithLastBlock set external last block height
+func (ob *Observer) WithLastBlock(lastBlock uint64) *Observer {
+	atomic.StoreUint64(&ob.lastBlock, lastBlock)
+	return ob
 }
 
-// GetLastBlockScanned get last block scanned (not necessarily caught up with external block; could be slow/paused)
-func (ob *Observer) GetLastBlockScanned() uint64 {
+// LastBlockScanned get last block scanned (not necessarily caught up with external block; could be slow/paused)
+func (ob *Observer) LastBlockScanned() uint64 {
 	height := atomic.LoadUint64(&ob.lastBlockScanned)
 	return height
 }
 
-// SetLastBlockScanned set last block scanned (not necessarily caught up with external block; could be slow/paused)
-func (ob *Observer) SetLastBlockScanned(blockNumber uint64) {
+// WithLastBlockScanned set last block scanned (not necessarily caught up with external block; could be slow/paused)
+func (ob *Observer) WithLastBlockScanned(blockNumber uint64) *Observer {
 	atomic.StoreUint64(&ob.lastBlockScanned, blockNumber)
 	metrics.LastScannedBlockNumber.WithLabelValues(ob.chain.ChainName.String()).Set(float64(blockNumber))
+	return ob
 }
 
 // BlockCache returns the block cache for the observer
@@ -170,37 +188,6 @@ func (ob *Observer) TelemetryServer() *metrics.TelemetryServer {
 	return ob.ts
 }
 
-// OpenDB open sql database in the given path
-func (ob *Observer) OpenDB(dbPath string) error {
-	if dbPath != "" {
-		// create db path if not exist
-		if _, err := os.Stat(dbPath); os.IsNotExist(err) {
-			err := os.MkdirAll(dbPath, os.ModePerm)
-			if err != nil {
-				return errors.Wrap(err, "error creating db path")
-			}
-		}
-
-		// open db by chain name
-		chainName := ob.chain.ChainName.String()
-		path := fmt.Sprintf("%s/%s", dbPath, chainName)
-		db, err := gorm.Open(sqlite.Open(path), &gorm.Config{})
-		if err != nil {
-			return errors.Wrap(err, fmt.Sprintf("error opening observer db for chain: %s", chainName))
-		}
-
-		// migrate db
-		err = db.AutoMigrate(&clienttypes.ReceiptSQLType{},
-			&clienttypes.TransactionSQLType{},
-			&clienttypes.LastBlockSQLType{})
-		if err != nil {
-			return errors.Wrap(err, fmt.Sprintf("error migrating observer db for chain: %s", chainName))
-		}
-		ob.db = db
-	}
-	return nil
-}
-
 // LoadLastBlockScanned loads last scanned block from environment variable or from database
 // The last scanned block is the height from which the observer should start scanning for inbound transactions
 func (ob *Observer) LoadLastBlockScanned(logger zerolog.Logger) (fromLatest bool, err error) {
@@ -219,7 +206,7 @@ func (ob *Observer) LoadLastBlockScanned(logger zerolog.Logger) (fromLatest bool
 		if err != nil {
 			return false, err
 		}
-		ob.SetLastBlockScanned(blockNumber)
+		ob.WithLastBlockScanned(blockNumber)
 		return false, nil
 	}
 
@@ -229,9 +216,9 @@ func (ob *Observer) LoadLastBlockScanned(logger zerolog.Logger) (fromLatest bool
 		logger.Info().Msgf("LoadLastBlockScanned: chain %d starts scanning from latest block", ob.chain.ChainId)
 		return true, nil
 	}
-	ob.SetLastBlockScanned(blockNumber)
+	ob.WithLastBlockScanned(blockNumber)
 	logger.Info().
-		Msgf("LoadLastBlockScanned: chain %d starts scanning from block %d", ob.chain.ChainId, ob.GetLastBlockScanned())
+		Msgf("LoadLastBlockScanned: chain %d starts scanning from block %d", ob.chain.ChainId, ob.LastBlockScanned())
 
 	return false, nil
 }
@@ -245,8 +232,39 @@ func (ob *Observer) WriteLastBlockScannedToDB(lastScannedBlock uint64) error {
 func (ob *Observer) ReadLastBlockScannedFromDB() (uint64, error) {
 	var lastBlock clienttypes.LastBlockSQLType
 	if err := ob.db.First(&lastBlock, clienttypes.LastBlockNumID).Error; err != nil {
-		// not found
+		// record not found
 		return 0, err
 	}
 	return lastBlock.Num, nil
 }
+
+// OpenDB open sql database in the given path
+func (ob *Observer) OpenDB(dbPath string) error {
+	if dbPath != "" {
+		// create db path if not exist
+		if _, err := os.Stat(dbPath); os.IsNotExist(err) {
+			err := os.MkdirAll(dbPath, os.ModePerm)
+			if err != nil {
+				return errors.Wrap(err, "error creating db path")
+			}
+		}
+
+		// open db by chain name
+		chainName := ob.chain.ChainName.String()
+		path := fmt.Sprintf("%s/%s", dbPath, chainName)
+		db, err := gorm.Open(sqlite.Open(path), &gorm.Config{})
+		if err != nil {
+			return errors.Wrap(err, "error opening db")
+		}
+
+		// migrate db
+		err = db.AutoMigrate(&clienttypes.ReceiptSQLType{},
+			&clienttypes.TransactionSQLType{},
+			&clienttypes.LastBlockSQLType{})
+		if err != nil {
+			return errors.Wrap(err, "error migrating db")
+		}
+		ob.db = db
+	}
+	return nil
+}
diff --git a/zetaclient/chains/evm/base/observer_test.go b/zetaclient/chains/evm/base/observer_test.go
index 49e0890e7c..402995367f 100644
--- a/zetaclient/chains/evm/base/observer_test.go
+++ b/zetaclient/chains/evm/base/observer_test.go
@@ -1,11 +1,204 @@
 package base_test
 
 import (
+	"os"
 	"testing"
+
+	lru "github.com/hashicorp/golang-lru"
+	"github.com/stretchr/testify/require"
+	"github.com/zeta-chain/zetacore/pkg/chains"
+	"github.com/zeta-chain/zetacore/testutil/sample"
+	observertypes "github.com/zeta-chain/zetacore/x/observer/types"
+	"github.com/zeta-chain/zetacore/zetaclient/chains/evm/base"
+	"github.com/zeta-chain/zetacore/zetaclient/chains/interfaces"
+	"github.com/zeta-chain/zetacore/zetaclient/config"
+	"github.com/zeta-chain/zetacore/zetaclient/context"
+	"github.com/zeta-chain/zetacore/zetaclient/testutils/mocks"
 )
 
+// create a temporary path for database
+func tempDbPath(t *testing.T) string {
+	// Create a temporary file to get a unique name
+	tempPath, err := os.MkdirTemp("", "tempdb-")
+	require.NoError(t, err)
+	return tempPath
+}
+
+// createObserver creates a new observer for testing
+func createObserver(t *testing.T) *base.Observer {
+	// constructor parameters
+	chain := chains.Ethereum
+	chainParams := *sample.ChainParams(chain.ChainId)
+	zetacoreContext := context.NewZetacoreContext(config.NewConfig())
+	zetacoreClient := mocks.NewMockZetacoreClient()
+	tss := mocks.NewTSSMainnet()
+	blockCacheSize := base.DefaultBlockCacheSize
+	dbPath := tempDbPath(t)
+
+	// create observer
+	ob, err := base.NewObserver(chain, chainParams, zetacoreContext, zetacoreClient, tss, blockCacheSize, dbPath, nil)
+	require.NoError(t, err)
+
+	return ob
+}
+
 func TestNewObserver(t *testing.T) {
+	// constructor parameters
+	chain := chains.Ethereum
+	chainParams := *sample.ChainParams(chain.ChainId)
+	zetacoreContext := context.NewZetacoreContext(config.NewConfig())
+	zetacoreClient := mocks.NewMockZetacoreClient()
+	tss := mocks.NewTSSMainnet()
+	blockCacheSize := base.DefaultBlockCacheSize
+	dbPath := tempDbPath(t)
+
+	// test cases
+	tests := []struct {
+		name            string
+		chain           chains.Chain
+		chainParams     observertypes.ChainParams
+		zetacoreContext *context.ZetacoreContext
+		zetacoreClient  interfaces.ZetacoreClient
+		tss             interfaces.TSSSigner
+		blockCacheSize  int
+		dbPath          string
+		fail            bool
+		message         string
+	}{
+		{
+			name:            "should be able to create new observer",
+			chain:           chain,
+			chainParams:     chainParams,
+			zetacoreContext: zetacoreContext,
+			zetacoreClient:  zetacoreClient,
+			tss:             tss,
+			blockCacheSize:  blockCacheSize,
+			dbPath:          dbPath,
+			fail:            false,
+		},
+		{
+			name:            "should return error on invalid block cache size",
+			chain:           chain,
+			chainParams:     chainParams,
+			zetacoreContext: zetacoreContext,
+			zetacoreClient:  zetacoreClient,
+			tss:             tss,
+			blockCacheSize:  0,
+			dbPath:          dbPath,
+			fail:            true,
+			message:         "error creating block cache",
+		},
+		{
+			name:            "should return error on invalid db path",
+			chain:           chain,
+			chainParams:     chainParams,
+			zetacoreContext: zetacoreContext,
+			zetacoreClient:  zetacoreClient,
+			tss:             tss,
+			blockCacheSize:  blockCacheSize,
+			dbPath:          "/invalid/123db",
+			fail:            true,
+			message:         "error opening observer db",
+		},
+	}
+
+	// run tests
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ob, err := base.NewObserver(tt.chain, tt.chainParams, tt.zetacoreContext, tt.zetacoreClient, tt.tss, tt.blockCacheSize, tt.dbPath, nil)
+			if tt.fail {
+				require.ErrorContains(t, err, tt.message)
+				require.Nil(t, ob)
+				return
+			}
+
+			require.NoError(t, err)
+			require.NotNil(t, ob)
+		})
+	}
 }
 
-func TestChain(t *testing.T) {
+func TestObserverSetters(t *testing.T) {
+	t.Run("should be able to update chain", func(t *testing.T) {
+		ob := createObserver(t)
+
+		// update chain
+		newChain := chains.BscMainnet
+		ob = ob.WithChain(chains.BscMainnet)
+		require.Equal(t, newChain, ob.Chain())
+	})
+	t.Run("should be able to update chain params", func(t *testing.T) {
+		ob := createObserver(t)
+
+		// update chain params
+		newChainParams := *sample.ChainParams(chains.BscMainnet.ChainId)
+		ob = ob.WithChainParams(newChainParams)
+		require.True(t, observertypes.ChainParamsEqual(newChainParams, ob.ChainParams()))
+	})
+	t.Run("should be able to update zetacore client", func(t *testing.T) {
+		ob := createObserver(t)
+
+		// update zetacore client
+		newZetacoreClient := mocks.NewMockZetacoreClient()
+		ob = ob.WithZetacoreClient(newZetacoreClient)
+		require.Equal(t, newZetacoreClient, ob.ZetacoreClient())
+	})
+	t.Run("should be able to update last block", func(t *testing.T) {
+		ob := createObserver(t)
+
+		// update last block
+		newLastBlock := uint64(100)
+		ob = ob.WithLastBlock(newLastBlock)
+		require.Equal(t, newLastBlock, ob.LastBlock())
+	})
+	t.Run("should be able to update last block scanned", func(t *testing.T) {
+		ob := createObserver(t)
+
+		// update last block scanned
+		newLastBlockScanned := uint64(100)
+		ob = ob.WithLastBlockScanned(newLastBlockScanned)
+		require.Equal(t, newLastBlockScanned, ob.LastBlockScanned())
+	})
+	t.Run("should be able to update block cache", func(t *testing.T) {
+		ob := createObserver(t)
+
+		// update block cache
+		newBlockCache, err := lru.New(200)
+		require.NoError(t, err)
+
+		ob = ob.WithBlockCache(newBlockCache)
+		require.Equal(t, newBlockCache, ob.BlockCache())
+	})
+}
+
+func TestOpenDB(t *testing.T) {
+	ob := createObserver(t)
+	dbPath := tempDbPath(t)
+
+	t.Run("should be able to open db", func(t *testing.T) {
+		err := ob.OpenDB(dbPath)
+		require.NoError(t, err)
+	})
+	t.Run("should return error on invalid db path", func(t *testing.T) {
+		err := ob.OpenDB("/invalid/123db")
+		require.ErrorContains(t, err, "error creating db path")
+	})
+}
+
+func TestReadWriteLastBlockScannedToDB(t *testing.T) {
+	t.Run("should be able to write and read last block scanned to db", func(t *testing.T) {
+		ob := createObserver(t)
+		err := ob.WriteLastBlockScannedToDB(100)
+		require.NoError(t, err)
+
+		lastBlockScanned, err := ob.ReadLastBlockScannedFromDB()
+		require.NoError(t, err)
+		require.EqualValues(t, 100, lastBlockScanned)
+	})
+	t.Run("should return error when last block scanned not found in db", func(t *testing.T) {
+		ob := createObserver(t)
+		lastScannedBlock, err := ob.ReadLastBlockScannedFromDB()
+		require.Error(t, err)
+		require.Zero(t, lastScannedBlock)
+	})
 }